Thanks, all, for the myriad replies.

After some other web research, I guess the biggest question at this point for "my friend" is whether it makes to lie on Customs forms re-entering the US.

I.e., they ask, "What countries have you been to?"

If you're not travelling with a license, you can say Cuba but refuse to give any more details. If they can't prove you spent money there--a technical violation of the embargo--can they still go after you? Any reason not to take the Fifth?

Of course, you could lie and just not mention the whole Cuba portion of the trip, but that's actually perjury--a criminal offense with possible jail time--and seems worse to me than some fine, however large, that's unlikely to be enforced.

I'm very curious about US citizens with experience on this front.

Thanks again!