Just drop the bomb, say hey I saw X at the movie on Friday.

Let it play out from there, he may have been with a niece or something innocent.

Or say nothing I prefer keeping my mouth shut and a lot of distance, I tattled once and still regret it....the fallout was they worked it out and I'm the biggest a-hole in the world, somehow the ***** managed to make it look like it was my fault she was screwing around.