I would leave it alone, unless both of them are close friends, but you say she is not. Getting involved in other people's matters always comes back to bite you on the ass. He may lie his way out of it anyway and then you're the one with egg on your face, made to look bad and everyone hates you. I have seen this happen many times, so for me if i know something and i am not that close to them i tend to stay well clear of other people's business.
Thinking of this i know of a guy who reported his two co-workers to the wife. He ended up having his house and car smashed in and eventually moved out. Think of yourself.
Paul