Found out about my girl's past

Confused

I have dated my girl for almost 6 months now. A few days ago, I found out that she slept with her cousin when she was 14 (The cousin being 20). I found this out from our mutual friend. I confronted her about this, and she said that she was raped by her cousin. It means that she has been abused, and I want to rectify this, if it is true. She is 16 now. I would really want her cousin to be punished for this. What do I do from now? She told me to drop it, and never mention it to anyone, But I have not slept yesterday night, and now I am at work, and can't stop thinking of it. The cousin lives in our town. Should I just go over to him, and deal with him, or report him? She was scared when I found about it, and I assumed her cousin must have threatened her to keep her mouth shut about it.. I need some help, as to how to act. She is young, and I really want to straighten things out. What if he does it again.

Thanks