What do I do when my bestfriend is endangering her life?

Okay so my friend who is 15 years old met some 20 year old guy on an app for itouch. She got really serious with him and has been in a relationship with him for months. They text and talk on the phone all the time and actually plan on meeting in person soon and getting married when she turns 18. She is planning on meeting him in person too this christmas. Me and another one of her friends think this is EXTREMELY stupid and dangerous. She doesn't realize how he controls her and thinks he's just trying to protect her but in reality he's just bossing her around. He makes her cry all the time because he ignores her for days if she doen't do what he says(not have friends over on certain days, not wear a certain pair of pants because he thinks they are too short). I think he's acting more like a father rather than a boyfriend. I hate the idea of her meeting him and thinking he wont hurt her. He might rape her and kill her! She plans on moving to the state he's in and living with him then finishing high school. We ve tried to tell her how stupid and dangerous this is and how she could end up dead if this guy is a freak. She gets mad when we try to tell her he's dangerous and he might not be who he says he is. I love my friend and I'm scared that she is gonna end up getting killed or raped. What do I do?