Well, I think I'd not trust her again. I'm sorry. While this does sound like it was in the past and she put the brakes on it once you found out BUT, it happened. You can't erase that. Some people that cheat do regret it when it ends their stable relationship. She either really needs attention and is willing to step out of the relationship to get it or she liked this guy even when she didn't want to, but either way, it happened. It's up to you if you want to rebuild trust with her but it will take time. And since she's never fully owned up to it, she's still not being honest.
So, if you are a young man, I'd go ahead and find someone you don't have this negative history with to have a long term relationship with. I'm really sorry!