You need to talk to her, not at either of your houses, because she might be worried that your parents will hear. go to like a playground, or a park, and ask her exactly what happened. You need to comfort her, even give her a hug if you have to. Tell her that you will be there for her no matter what happens. Tell her that she needs to tell her parents, and that she should press charges. Make sure she knows that what they did was wrong, and that she had the right to say no. Also tell her that no matter what the situation was, it wasn't her fault, and that they were very wrong to do what they did. Tell them that if she doesn't do something about it, then it's like being on their side. Tell her that she needs to do whats right, and by standing up for herself, she will be standing up for someone else, their next victim.