Forget about her.
Why would you want someone who cheats. Obviously if shes cheating on this guy shes able to cheat on you too if you get into a relationship.
I would stop contacting her now and forever. I personally would move onto the next girl and probably help her boyfriend out.
I'm not sure if your willing to do this nor do you know the person because you didn't give me enough information. I would contact the boyfriend by a message or something and tell him that his girlfriend has been up to when he wasn't around. Seeing him in person could be confrontational, and it's up to you personally if you wish to do this. But if anything leave him a message of what's gone on and say your sorry, you didn't know, and you don't want anything with her now.
That's totally up to you, you didn't really do anything wrong.