I know that you must be very worried about your friend and you have done the right thing by trying to persuade her to reconsider what she is doing and to stop having sex with these boys. But I think it will take more than your e-mail or talking to her as a friend to stop her self-destructive behavior. You need to strongly consider talking to your parents about her behavior and a counselor you can trust. Maybe there are older sisters or brothers in her family that you can talk with. You could also offer to attend counseling with her to for support.
You know that your friend is not having sex with all these older boys because she loves them or has wonderful, respectful relationships with them. There is something seriously wrong with your friend emotionally that is forcing her to act in this inappropriate, risky, shameful, self-hateful way. You can be her friend but you can't save her from this destructive path she has taken all by yourself. Please talk with some or all of the people I suggested and also get some of your mutual friends together with her to try to find out why she is behaving like this and to tell her that you care for her too much to keep letting her do this. She may tell you that you're not her friend anymore, but you have to be brave and think of her safety and health above any hurt feelings you may get from what she tells you. Please let me know what happens. I'm hoping you can summon up the courage to tell the right adults about this because she is in big trouble that could affect her entire life.