kill his dog, fuck his sister and burn his house down.
but for real though, i would probably just do nothing.
Gathering info for the police will just be a waste of time because cops don't care about shit like this..
You could always find out where dude lives, follow, and confront him out in public somewhere and be like what's up buddy!?! and take it from there, if your a tough guy he will give you all your shit back while begging you not to fuck his girlfriend.
Or you could pick thru dudes trash, get his DNA and set him up.
but Your best bet would be to run up on dude with a louisville, wearing a ski mask while he paints one of his e2e burnerz.