Actually, it's not that hard. I cook Japanese about once a month, sometimes more often. I lived there for two years when I first transitioned, and I love the cuisine. Next to authentic Mexican, it's probably my favorite.

You don't have to go the full Kaiseke banquet route: as someone else suggested, that's a bit over the top (for one thing, you need to have the right plate for each ingredient - Japanese food is as much about presentation as taste). For another, it would cost a fortune to assemble all the ingredients.

Small portions are de rigeur. Kaiseke consists of about 15-30 very small dishes - usually no more than a mouthful each (and hence the reason it's expensive to do for only a few people). A regular Japanese meal consists of soup plus three dishes (plus rice and pickles). If someone wanted to really go to town to impress they might run to four courses. Only in a formal banquet would anyone have more, and as others have noted, for banquets, you eat out.

If you're cooking at home, you want to stick with izakaya-style recipes - literally, bar food. If I'm going to do Japanese, this is what I cook, and when I lived in Japan, the few times I got invited to people's homes, it's the kind of thing they cooked. Being invited to a person's house is a big deal in Japan. I lived there for a year before I ever got an invitation.

Even if they cook Izakaya style, they'll still be small portions if the person is Japanese. Bigger if they're Japanese-American.

My Japanese language skills suck but I still remember how to cook. :)

Suggested menu

When your friends first sit down, you'll want to give them something small before they even get the appetizer. Maybe some edamame (green soybeans in the pod, steamed and sprinkled with salt). Serve them in a bowl with an accompanying bowl for the discarded pods. You can buy frozen edamame at an Asian food store.

Appetiser:

A small plate of sashimi. This has to be cut almost immediately before serving, and your fish has to be caught-that-morning fresh. In Japan, people will cut sashimi from live fish. Sea bass, salmon, tuna, or if you want a very strong flavor, mackerel. The pieces will be sliced extremely thinly, with an ultra-sharp knife that is never used for anything else. Top with some fresh salmon roe (which is like a red caviar) and maybe a little fresh sea-urchin if you can get it.

Sashimi is utterly pointless unless the fish is extremely fresh. And cutting sashimi is something that takes a lot of practice, but that shouldn't be a barrier if your character is Japanese. Even in Japanese restaurants here it's hard to find anyone that cuts as thin and evenly as good Japanese chefs do (maybe I just haven't found the right sushi bars here). It's worth considering, though, that most of the people you'll see making sashimi or sushi in Japan are men - as far as I can tell there are no female sushi chefs. Obviously women know how to cut fish, but it is something that takes time to learn properly, and I was taught how to do it by the father of a friend.

You could make sushi instead (nigiri-sushi is the kind where the fish sits on top of little pads of rice), but then you have to be very practiced at adding exactly the right amount of rice vinegar, sugar and salt, and skilled at pressing the rice with your hands. Sashimi is actually simpler, so long as you're skilled with a knife and know your way around a fish.

I'm okay with Sashimi but I never seem able to get the rice exactly right for sushi.

Entree:
Grilled salmon. If you don't want to do salmon any good grilling fish will be okay - oilier fish obviously grill better. Either small sections of Salmon belly or tail, skin still on, or salmon heads, sprinkled liberally with salt, and then grilled over an open flame (I have a small hibachi barbecue on my balcony I use for just this purpose). Yes, the head of a fish is something people in Japan will eat. The cheeks of a large salmon barbecue very beautifully, even if there's very little fish on the bone. If you're entertaining guests you'd probably stick with the belly or tail sections. But fish heads make good eating! When cooking the chef will probably baste with a bit of mirin and soy sauce (the mirin cuts down the taste of the oil in the fish).

Accompaniment to the fish: grilled eggplant with sweet miso sauce and sesame seeds. Slice the eggplant into 3/4" slices, cross-hatch them with a knife, salt them for 30 minutes then drain the bitter liquid. Then brush them with a little vegetable oil. Grill the slices, then when they're beginning to brown, brush them with a sauce made from dashi, mirin, cornstarch, plus a little sugar and some white miso paste, maybe a dash of yuzu kosho (japanese pepper) as well. Stick them back on the griller so the sauce caramelizes. Sprinkle with lightly toasted sesame seeds.

Actually, at an izakaya they grill pretty much anything: shrimp, corn, even sliced potato. Everything gets a dash of sauce or soy and cooked over the barbecue.

For balance, to avoid everything being grilled, you would normally consider a side dish of something like sunumono, which is cucumbers and wakame (an edible seaweed) marinated in rice wine vinegar.

Every meal - even the cheapest - gets served with miso shiru (miso soup: made from dashi - fish stock, which a good Japanese cook would make herself but which can be bought in a store - and miso paste - available at most decent Asian grocery stores, or you can buy it online. Include some thin-sliced scallions, maybe some mushroom, some diced tofu, possibly a small piece of shrimp and even a (tiny) touch of grated ginger and maybe a dash of soy.

Side dishes: Japanese pickles - you can get these online.

Throughout the meal there will be copious amounts of rice. Japanese rice is a different kind of rice to the stuff we get here. It's a short-grain, round rice, which is quite sticky. You can't do Japanese food with American rice. Fortunately, like mirin, dashi, etc, it's readily available at Asian stores, and online.

The Japanese don't always do sweet desert at the end, but when they do it's as like to be something like konnyaku, which is a fruit jelly. Again, you can buy the jelly powder online. It's a kind of agar. Add a little castor sugar, but not too much. Berries are good done this way.

In terms of drink, at an izakaya you would usually drink beer, but cold sake is also acceptable. Hot sake is a matter of taste - I think it's an abomination, but a lot of people who aren't used to sake prefer it.

As alternatives to the fish, you could do something like tempura, but that's very hard to do well (the secret is to keep the batter very cold), and it's not good for entertaining, because the cook has to stand by the fryer to do each ingredient one-at-a-time, so they don't get to sit with everyone else while they eat. Tempura is an eating-out kind of experience. Likewise, you could make yakitori (meat, tempura and vegetables on skewers, basted and grilled), or even shabu-shabu, which is an easy one-pot dish of stock, vegetables, tofu/meat/fish and egg, but again I think that's pretty much an eat-out deal.

A more likely alternative might be teriyaki chicken or beef. Teriyaki sauce is soy sauce and mirin, with small amounts of ginger and garlic. You might marinate the steak or chicken first, and you'll baste it continuously while it's on the grill. You would never normally serve more than about 4 ounces of teriyaki beef or chicken to anyone, and you would slice a steak or chicken breast into thin slices before serving.

Tonkatsu, which is breadcrumbed pork cutlets, is easy, but I wouldn't do it as a fancy meal. Fish is safer. I've always consider tonkatsu to be best for drunken salarymen. It's too greasy.

If, for the sake of your story, it has to be a banquet, then the Wikipedia page for Kaiseke is reasonable informative, and contains a sample menu: http://en.wikipedia.org/wiki/Kaiseki. Be aware though, that in Kaiseke, each dish will contain only the ingredients that are in season, and requires, to be formal, a separate plate or bowl for each course. The courses are served in sequence, over a period of hours. Because of this, you'd normally never get that kind of food in a home - you'd never see the hostess.

After dinner the men will probably get shitfaced on shochu (rice/barley liquor, much stronger than sake) or Japanese whiskey. The women, unless they want a reputation, will stick to sencha (Japanese tea made with green tea leaves - different to Chinese green tea, in which the leaves are often fried before drying). Japanese tea should never be made with boiling water - the water should be not quite boiled). If it's cold, serve the tea hot. If it's summer, make the tea in advance and serve it chilled. Serve the tea with little Japanese sweets made from bean paste - they're nicer than they sound.

Gaikokujin (the polite term for foreigners, as distinct from gaijin, which is pejorative) are often intimidated by Japanese food but it's actually very quick and easy to cook, incredibly healthy, and it doesn't leave you feeling bloated. :)

If you have more questions please feel free to ask. I'm not online much at the moment but I'll do my best to get back to you.