Call the police, drag your lying, cheating girlfriend through a (hopefully) lengthy court case to convict your "rapist" friend.
Worst case scenario, they believe your girlfriend and your former friend gets a few years to think about why you don't fuck your mate's girlfriend.
Best case scenario, your girlfriend crack under the pressure and confesses that she made it all up, therefore learning a valuable lesson about lying. Hopefully this will incur charges on her. Bring popcorn.
Enjoy! And remember, you're only doing this because you love her and can't stand to see the bastard responsible for her pain go unpunished...