If I read you right, you want to introduce your chum to offal in New York. You are talking my language.
Here are some favourites. Menus change, of course, so you might want to check with the restaurants:
Brains in black butter and sweetbread dishes are often on the menu at the inexpensive La Lunchonette in Chelsea. Same organs are also among the specials at Alfredo's, but they had sold out when I tried to sample.
Several black pudding/sausage options. Les Halles on Park Avenue usually has it, but I find their version a bit soft and runny. Better experiences at Chez Josephine on 42nd and Florent in the meatpacking district. You will find variations on the dish in Philippino restaurants such as Cendrillon and Latin restaurants such as the Old San Juan on 9th (or Old San Juan Too on third). The San Juans also have good grilled veal sweetbreads.
At this time of year, I hear the oxtail at Blue Ribbon is excellent, but I haven't tried it. Lots of upmarket restaurants venture oxtail ravioli, but that's a bit wimpy, right?
Cheap and delicious pig spleen sandwich at a storefront sandwich bar on First Avenue called La Focacceria. Can't find the address - it's a few blocks up around fourth street.
Ever eaten duck's blood? Rush to the Malaysia restaurant at 48 Bowery (it's actually down an alley way leading west off the Bowery just north of Bayard). They do duck's blood (which is like liver-flavoured jelly) about twenty different ways, and everything else I've tried there has been cheap and delicious too.
One of the most exciting offal experiences in the five boroughs is buying frituras (tongue, pig's cheek, intestines (no, not tripe, intestines), morcilla, etc) from one of the Dominican vans parked in the Bronx. That's a tough assignment unless you're a confident Spanish speaker (and okay with hanging around the Bronx after dark), but I'll look up a location if you're keen.
If you want something more elegant, consider L'Absinthe, an expensive Upper East Side bistro which used to serve sensationally good calve's head with a leek salad. Last time I was there they'd replaced it with a fancied up stuffed pig's feet, but it wasn't hearty enough for me. (They do offer whelks from their raw bar - not offal, but I suspect appeals to the same sophisticated palate.)
Generally, Chinatown is a good and obvious bet. Your only problem is going to be whether the offal dishes are listed on the menu in English. Puerto Rican and Domincan restaurants (try the Lower East Side or Alphabet City in Manhattan) usually serve mondongo, the tripe stew.
I have the impression you are not planning on buying and cooking yourself - that would be a whole other list. Kidneys are not readily available, and what I have also not found in New York are testicles (or rocky moutain oysters, if you insist). If anyone knows where to buy or eat them, I'd be most interested.
(Edited by Wilfrid at 11:57 am on Nov. 8, 2001)