Ex is a pedophile! What should I do?

I recently had a 15 year girl tell me that she had sex with my ex who is 23. She told me because he had lied to her and she found out I was an ex on FB. She didn't know how old he was at the time because he had lied about his age and pretty much every detail about himself. She wanted to know the truth. Now I feel like I should report it. She doesn't want me to and in fact begged me not to. What should I do in this situation. I know she is telling the truth by the details of the inside of his car. What should I do?