shes a big slut(no one can deny that), the best thing you can do is either:tell her bf and part ways or dont tell her bf but still part ways, if you dont like what she is doing now, then gtf away from her. shes going to end up with crabs and std anyway, so you are better off with a friend which is up to your standards.
but if you want some casual sax, ask her, i bet she wont mind.