If her boyfriend is saying these things to YOU, then I would believe her when she says that he FORCED her to have sex!! It sounds to me like he actually RAPED her.

She should have told someone about it when it happened, and stayed away from him completely! I'm sorry this happened, but it sounds like it really wasn't her fault.
This is all in the PAST. What happened just plain happened, and you can't keep blaming her for something that sounds like wasn't even her fault. She's very sorry about it -- don't keep making her feel even worse. She's probably dying inside, thinking that YOU think the very WORST of her -- and there's nothing SHE can do to fix the past. No one can.
When you hold resentments like this, all you're doing is hurting yourself. You're eating yourself up on the inside. Wouldn't it be easier to forgive her? And don't you think she really deserves it? I do. I think you need to forgive her and NEVER bring it up again!! And if you two have a fight in the future, you CANNOT bring that up and throw it in her face!!! That's a rotten thing to do. So if you go back, you have to PROMISE NEVER to bring it up again.
You two deserve to have a happy life, and you can if you just plain forget the past. If you two go to church, talk to your pastor, or religious leader, and see what he says. But stay away from her ex -- he's a bad man! God bless and please take care.. Hugs, Lee