Im sorry but once someone has abused your trust once things are never quite the same again. If you think he's generally sorry for what he's done and that there might be some hope left for the relationship then by all means work on it - but i think this would only apply if he'd cut all contact with her. The fact that he is still seeing this person AFTER the affair with her says that something isn't quite right. It doesn't sound like he has a lot of respect for you...