Lots of points have been raised.
For the victims of sexual abuse, nothing makes it go away. It's a scar they carry for life. Many end up being abusers themselves. There's not much I can say on that account. Just stand up and be there for her. She probably doesn't need some masculine show of machismo. She probably needs a shoulder to cry on and a hug that'll make her feel safe. It's OK to be angry and pissed off. But in front of her you need to stay calm, be in control and remember that SHE is the one that's dealing with it, you're just watching her deal.
Double jeopardy means he can't be tried twice for the same crime. But each incident would count as a separate crime. 6 victims, 6 crimes. 1 victim 6 times, 6 crimes. There are two reasons you can't be tried for the same crime multiple times. The first is that if you are tried and get off, they could just keep retrying you until you were convicted. The second is that if they could just keep retrying you, they wouldn't have to take a lot of time to build the initial case.
Under our system they basically need to do it right the first time because it's there only shot. It's also why most criminals face many many charges but are only ever tried for a few. That way if they can't nail you for the first few crimes they always have a fall back option. They'll usually try you for the bigger crimes first, and if that fails they can go for the lesser shit.
The reason you shouldn't advocate vigilante justice is because it doesn't work. More often than not innocent people are attacked by vigilantes. A 12 year old is pissed off at dad so she decides to claim sex abuse and the vigilantes kill daddy. Thing is daddy never did it, she just wasn't allowed to play XBox. (It actually happened minus the vigilantes.) A man is accused of rape, and the vigilantes castrate him. Except it wasn't him. The law should be an emotionless bitch. It needs to be cold, calculating, contemplative and above all not reactionary in the heat of the moment.
Can you rehab an abuser? That depends on the abuser and why they did it. Some are just sick as hell. But what about the ones that were abused themselves? Study after study has shown that children that are abused may often times become abusers in their later lives simply because it's what they now. There's a rather large difference in my mind between someone who gets his rocks off raping children and someone that frankly doesn't know better because that's how it was when they grew up. The crime is still heinous but the motivation behind it should be taken into account.
I know that when you're faced with things like this it's hard to remain cool and detached. It's easy to let your emotions run over the top of you. But justice is never served when you're not in control of the emotions. Take a step back, try to remove yourself from the situation (mentally) for a minute and see if you can't look at it in a different light. In this case try to remember that it's not about you or your reactions, it's about the girlfriend and what she needs from you.