Sexual Assault Help

Please help. My 21 year-old sister lives in Nashville and she just told me that she was attached last night by a guy she met through friends. She went over to his house where there were a bunch of drunk guys and he grabbed her, pulled her into his room, choked her, tried to rip her clothes off, and tried to rape her. Thankfully, she kicked him in the groin and managed to escape, but he's been contacting her ever since and she refuses to report him. He also has a history of this type of behavior and has been accused by other women, including my sister's roommate. But my sister insists that she can't go to the police. How can I convince her? What should I do?