American food to feed to a British friend?

[veti]

Peanut butter is crushed peanuts... I don't really see how it could vary that much.
Sometimes it's sweetened... which, again, doesn't seem like it would vary much.

You'd be surprised. Some cheapskate manufacturers add extra vegetable oil. Sometimes, as you've noted, they add sweetener (which makes the whole concoction vile beyond belief, in my opinion). Most, but not all, recipes add salt. The variations can be quite noticeable.

(Edit: I'm slightly surprised to learn there isn't a "peanut butter with added chili" on the market. I'd try it. Just throwing that out there, in case anyone reads it.)

As for fried chicken... I get the impression it's just as common there as in the united states? Stuff like KFC and the british chain "Nando's".

Point of order: Nando's is a South African chain, although (according to Wikipedia, at least) they currently have more branches in the UK than any other country. However, their signature chicken is "flame-grilled", not fried.

Chili, at least home made chili, is something my boyfriend has been exposed to at least.

Yeah, chili is ridiculously easy to make at home. At least in some form, following any one of about 4,027,911 recipes you can find online from any country of your choice. If you know a restaurant that makes a particularly good/distinctive version, then sure, it might be worth taking him - but just standard homemade or cafe/deli chili? Not gonna impress.