I NEED HELP!!! PLEASE!!!?

Okay. This is a little long. I have a really good friend of mine who had a bad breakup with her boyfriend in November. She is 17 as of early April, he is 18 as of about the same time. The entire time since the breakup, this guy hasn't left her alone, always pulling her aside to talk to her, sending her texts and calls nonstop, even when they're in the same classroom. It's gotten to the point where their little "talks" end up in her bawling her eyes out and he's cursing at her saying it's all her fault. Everyone agrees, aside from his closer friends, that he is just being a b*tch. Well, I've learned that a few weeks ago, he began blackmailing her, threatening to tell her mom, who is extremely strict, about her occasional marijuana smoking, her sex life, and more. His condition? She do anything he wants. This went for nude pictures, random pickups in his car, and over April Vacation, he forced her to have sex with him, even though she is in another relationship. I learned about this today while hanging out with her. I don't know about you, but that sounds a lot like rape to me. Now, I want to beat his face in until it's unrecognizable, even though he's older, bigger, and probably stronger, because this girl is like my sister. What should I do? And what should she do? I need help, because me, her, and him have a Phys Ed class together, and I feel I'm going to rip him apart the next time I see him.