My vote is don't tell, especially if you don't know the person you're telling that well. In my opinion, there's already something wrong with the relationship if there's cheating going on. Fortunately, I haven't had to deal with that yet so I can't speak from personal experience, but one of my closest male friends is the concierge at a very nice LA hotel and the stories he tells me are
Obviously some of the clients who register or "keep" women there, celebrities included, are married men because they register under a different name and usually pay with cash. I always feel really bad for the wives. But his job requires him to be discreet and I think the same rule applies to someone you know as a friend or an acquaintance. If you don't know all the facts about the relationship, it's better to leave it alone I think.
