Who's at fault depends on how it happened. I'd imagine the girl, being the one who's attached, is more likely to be, but I could think of situations where the bloke might be. If it's a one-time mistake, definately stay out, and let them forget it. If there's some sparks there, talk to them, but stay out initially. If it only happens once, it's not your concern. If, but only if, they start having a relationship on the side, then her boyfriend needs to know.