Hey Emi,
That's the main problem with Sexual Abuse cases, either there's not enough evidence or there's no evidence, and they won't convict someone without evidence. However, she can report it to the police and have them make a record, so that if he does it again, to her or another girl, there will be a trail, proof that he's done it before and that he'll keep doing it.
Could you get your friend to talk to a counselor? A counselor might be able to help her deal with what she's going through right now, and can actually help her heal. You sound like a really good friend, to be so concerned for her, but don't forget to take care of yourself too.
If you need anything, don't hesitate to
pm me.