I don't know if you have personally talked to the family... but if they are close to you maybe talk to the parents alone. If they know how close you are to their daughter than they will know that you are being serious... Tell them how worried and serious this issue is and if they still don't believe you than tell them to get their daughter checked out by a doctor and they will know if she was raped or not. I am sorry if this is not at all helpful... I don't know what else to say... if nothing else just continue being a good friend to that poor girl.
PM me if you want to talk.