What should I do about my friend whose going out with an older guy?

Okay, so my friend and I are both 14 but I’m almost a year older than her so I play the big sister role. A few days ago, she personally met this 17 year old guy whom she’s been talking with for about 2 months through AIM/Instant messenger. The first time they met, she told me that he was so strong and she kissed him and they made out for a long time. She was a little scared since it was her real first kiss. The next day she cut school to go “watch” a movie in his house (I’m from the side of NY where when you go to a guy’s home, we know exactly what’s gonna happen. You know what I mean). So anyway, she went. I then went to her house to ask what happened. When I saw her, I noticed a HUGE hickey down her neck and her lips were swollen. She was acting so strange and then she wouldn’t tell me nothing except that the guy tried to push her into having sex with him but she said no. I also found out that this 17 year old guy only goes out with 13-14 year olds. I’ve figured that maybe he goes out with girls in that age group because maybe he thinks he can tell them whatever he wants them to do. What should I tell her? because ever since she’s been going out with him I’ve been worried and I’ve told her that. What can advice or tell my friend?