I probably would not have asked the guy whether he had been upgraded but, if I happened to find out that he self-upgraded, I would have informed the FA, just as the OP did and just to spite the guy.

That said, the FA's always discover self-upgraders before departure and send them back from whence they came. I've witnessed this myself on several Hawaii flights (see the thread I started on self-upgrading that I can't find with search right now).

And, some people mentioned fearing reprisal from the self-upgrader after arrival. Why would anyone be afraid of some loser self-upgrader beating them in an airport full of witnesses and federal law enforcement? Most people talk the talk, but 999 out of 1000 wouldn't have the gumption even to consider following through on a threat (battery charges are a big deal).