How to beat up a guy who touched my sister?

Okay I'm a girl, I'm 14. And my older sister is 17, she has a friend who is 16 and they're both girls. They ALWAYS hangout together, they're practically sisters and I guess they hungout with this guy and the guy ended up touching my sisters thigh & crotch. My sister has a bf and she pushed his hand away and her bf picked her up and she was crying. I KNEW from the begining this guy was UNTRUSTWORTHY. The guy & my sis are somewhat close because they're making a band together. I KNOW PEOPLE WHO CAN FXCK HIM UP TILL HE DIES, but I'm not doing that. So how can me & my sisters friend beat him up? He's like 18-20 I forgot. I was thinking a bat, but do you guys have any good ideas? Don't tell me to go to the cops or whatever cause no way in hell is that happening. This happened like at 3:30 am so no authorities. I want to WITNESS HIM SUFFER. Thank you!