Blake, I know I’m chiming in late on this but these guys are right. I’ve been there man, except it wasn’t some guy I didn’t know, it was my former best friend with my (now ex) wife (in addition to another few mind fucks she put me through). Plus if you take her back, there’s a damn good chance she’ll do it again (again, my ex cheated on me after we tried to work things out, burn me once shame on you, burn me twice shame on me)
As much as you want to beat the hell out of this guy, it’s not worth it. If you show up after work with your boys, then you’re asking for charges, and “Well he did my girl” isn’t a good defense in most of the courts (although I kinda like Jack’s first idea). 
You need to focus on school, lifting, sports, music, anything you need to get her off your mind. Take all of that anger and hatred you feel and channel it into making yourself a success. Success is always the best revenge, while she’s still working in some store, move on and become a successful powerlifter, or lawyer, or businessman or doctor.
Let her go and move on, find someone else, either Miss Right or Miss Right Now.
Peace out.