Am I right to feel annoyed?? Cheating qu...?

no he shouldnt tell her. It'll only ruin what they have. And by the way its him who's the knob for cheating not you for falling for someone who was taken. He'll cheat again and she'll find out. These things always come out. But if he tells her because of you then you'll be the bad guy not him and he'll end up hating you. If he ever comes back then you dont wanna ruin your chances of another dirty weekend ;)