How do not loose my close friend? This is very depressing... I need urgent help?

Ok so there is this girl i dated for about 3 months. We decided not continue dating because it was kind of awkward and instead just become very very very close friends. like i trust her more than anything. i still love her and care about her and stuff, but only 2 days later, this really annoying fake, but seductive guy that everyone at my school hates picks her up. One of my other best friends who is also very close with her basically got alienated by this guy, and is now turning back to drugs, which she had led him away from. he sees no point in listening to her anymore. Hes trying to do the same to me and its really awful because hes manipulating my friend. I cant fight him because it would make me look bad if i strted it. hes making her change and stuff too. We try to tell her it needs to stop and her girl friends tell her the same thing but she gets really upset and tries to defend him. she exploded in the car the other day on my other best friend about him. hes stressing her out and making her upset. shes changing. People are losing her friendship and its making everyone sad. THis guy needs to die. Anyways, im really afraid of losing her as a close friend and i want to become closer friends at the same time. i want them to break up so everything stops falling apart. I dont want her to be changed into some fake goodie he's making her. she was once the realest person i knew. What do I do???