Ooh, I hadn't even thought about whether or not Joel might use Yiddish terminology in conversation (the "mensch" thing is only in his head, and Joel had been kind of floored by this other person's kindness, so I think it's probably appropriate in context given what you've said?).

But it's possible (probable, even!) that the question of whether Joel uses Yiddish terms when talking to his non-Jewish friends will come up later in the book, so thank you very much for that extra info that I hadn't even considered. :D