Sounds like your friends are either jerks or they were just screwing with you.

Sneak into their homes in the middle of the night and scare the crap out of them. Then threaten their lives and tell them not to cruise your woman again - or else. It also helps if you break something valuable, for effect. BTW, Wear a black leather jacket whilst doing all this.
If they apologetically talk to you afterwards, they're your friends. If not, get new friends.

Or better yet, have you tried to talk to the woman in question? Does she want you?