I'm with Descent and Icarus on this one. There was a guy in my town who was arrested for rape last year and after the news broke, two other girls came forward with the same story. Turns out they had been too afraid/ashamed to say anything before. Thing is, if they had, maybe he wouldn't have had the chance to do it 3 TIMES!!
I won't even pretend to understand how your friend must be feeling, but I would assume that she will harbour more than a little anger towards the guy who did this to her. She could do herself, and possibly future victims, a big favour by stopping him now.
The right thing to do is not always the easiest.