What do I do do in this situation (by friday!)?

My best friend, mel, is friends with a girl named bobby. who i hate. she doesn't go to our school anymore, but she is with a 19 year old (she is 13!!!) she is trying to hook up mel up with a 17 year old, but mel is only 12... they talk a lot and are going to the movies on friday but i think hes gonna just make out with her!!! i tried talking to mel about it but she thinks she can protect herself but what if he tries to her or something?????? I just don't want her to get hurt.... What do i do????