What do I do in this sticky situation?

My best guy friend had sex with a girl in our grade, and another girl is telling everyone. I love my friend a lot and I really want to protect him. I really want to confront the girl and tell her to stop telling people, but I feel like maybe I should just stay out of it. My friend said it was okay for me to talk to her (actually he wants me to slap her across the face), but I am not sure if I should… any advise?
***AND NO, I WILL NOT TELL AN ADULT***