Don't get physical
OK, we'll put it out there. If you round-housed this pretender to the face, you'd be completely justified in doing so. We wouldn't blame you, and truthfully, neither would he. But rather than going the expected Neanderthal route, try taking the path less followed and be more composed than expected.
Hear us out on this one. Sure, you won't get the satisfaction of making fist to face contact, but you will retain your gentleman card and your composure. Plus, any injuries your boy retained would be an indelible mark of your jealousy and subsequent revenge. He'll feel like he's endured his punishment and is absolved of sin, and she'll feel smug you give enough of a shit to kick your friend's ass over her.
Don't give either of them the satisfaction. Let them wallow in self-afflicted guilt, preferably forever. That's more painful than any knock to the dome. Trust.