Oh I think this one is pretty easy... having worked in that industry, I'd guess that this guy did something that's both common and a big no-no. If you were able to catch 1. the name of his employer, and 2. any one of the parties involved in the transaction (or any identifying information at all), you should call the compliance office at his employer and let them know.

"I was sitting next to this guy on flight so and so and he was going on for 90 minutes, swearing about whether or not KKR was going to up their offer from Toys R Us... dude looks like [...] and I was wondering, does this represent an official recommendation from your firm that Toys R Us is on your 'buy list'?"

This was one thing they hammered in to us - don't talk about real stuff even in an elevator if you think you could possibly be heard. People get fired for even inadvertent disclosure. Of course, that presumes you get caught, and fat-head "masters of the universe" never think they'll get caught (see: Mike Milken).