Date ?

This is very important and she may not realize it but you need to sit her down and tell her that you love her as a close friend and tell her that you would never want her to be hurt. You need to bring the said events to her attention and make sure she understands the reality of what he did at the frat party and that it was not right. I wouldn't say go to her parents unless you think there is no other way to keep her safe...but at the same time in doing any of this she may become VERY upset with you. If that does happen which it probably will, don't take it personally, she just doesn't want to admit that what happened to her was wrong and that this guy is a horrible person. Just remember...tough love is the best kind of love in this kind of situation. Otherwise...try to convince her to talk to her mother is they are close...let her know that talking to her mom is probably the best option she has.