First of all, I'm so sorry for both of you to be in such a horrible situation.
Do not blame yourself. The only person at fault for this is the scumbag who raped her.
If I were you (of course I'm not saying this is necessarily the right thing to do)
I would try again to convince her to tell the police. Perhaps if you explain to her that turning him in could protect other girls from going though the same trauma, this might sway her. I would also try to convince her to see a therapist or something along those lines. It's very common for rape victims to feel guilt, disgust, and a range of other emotions as a result of what's happened to them. It's likely she will need help to recover from this.
If she won't go to the police, and you can't tell anyone but are still worried about the party. I personally would go myself and watch the guy like a hawk. If he makes a move on a girl, follow him. Be there to stop something happening if there is no one else to do so. Again, this probably isn't the best plan of action because it'd put you in danger- but it's what I would do.
Also, I realise this is a sensitive question and would be the last thing she wants to hear- but I assume the guy didn't exactly use protection. Has she tested for STIs or pregnancy? Just because the risk is there and the sooner any potential issue is spotted, the quicker she can get treatment for it. Tell her that he shouldn't be allowed to damage her body any more than he already has.
Ultimately, I think you both need to sit down and really talk to each other. It's clearly eating you up inside, and I imagine she feels even worse. It may take a while. But I think you both seem to need to come to terms with what happened, and help each other to heal. You for the guilt you feel, and her for whatever emotional demons she's burdened with and other side effects of this.