First of all, if you and your friends are smart when you find out who it is, DO NOT BEAT HIM UP. I REPEAT, DO NOT BEAT HIM UP. You won't help her case if you do so. As horrible as what he did is, it's not a good idea to do so. My ex-gf was raped at gunpoint, we found out who it was and went after the guy. Afterwards when we talked to the lawyers they told us that it was really dumb and he could end up getting a lesser sentance, or even get off because of it, which he did.

By beating this guy up if and when your friend presses charges and tries to get a conviction, it will hurt her case if the guy was attacked by her friends.

So trust me, from experience, don't do shit to the guy just let the cops take care of it. But if the guy does get off, then you can beat his ass, just give him his day in court first. Don't be dumb about it.