You should marry him. That'll teach her a lesson...that she won't soon forget. Hah!

Confront her face to face and ask for proof, she may be lying to sabotage your relationship... If it is true, well... Bullet dodged

This is half an FML. On one hand, you got cheated on. On the other hand, he's obviously such a massive douche, that you aren't losing anything worth having.

I could never understand why two supposedly sane women would fight over a person who was cheating on both of them - But then I’m not a fan of Jerry Springer... There is some possibility that this bimbo is just trying to break up you and “boyfriend” for her own reasons. Find out what you can about her and confront “boyfriend” (if he’s not already an ex by now). If he admits to this or if his stories don’t add up drop him like the POS he is and don’t look back. If that’s the case you are better off without him.