Best friend slept with girl of my dreams, what SHOULD I do?

[Burdzthewurd]

Backstory: I've been best friends with this guy since freshman year in college (we'll call him John). He introduced me to his then-girlfriend's little sister Anna. I am 19 at time, she is 17. We hit it off but don't see each other that much due to distance (2 hours or so). Years pass and we grow closer as friends with some innocent flirting or so.

It's 3 years later, and she comes to meet me at my best friend's flat for the night. We both had feeling for each other, but everytime we were alone, we would get close to each other, then she'd pull away and start drinking again (she's really drunk, I am intoxicated but not beyond acting myself). My best friend is buzzed but not drunk. He tried to give us alone time to no avail.

We all head to bed around 3. I get up to take a piss and notice the door to her guest room is open and she isn't there. I go to the bathroom, which is next to my best friend's room. I hear them both moaning loudly. Obvious what's going down.

I'm not a violent guy, and beating his brains out won't do anything for me emotionally. What sort of approach do I take in confrontation though? Yes I'll listen to what he has to say, but nothing justifies what he did. Maybe it's stupid since I don't ever plan on talking to him or her or anyone they know again, but letting him off the hook is unacceptable, I'm only human.

What would many of you do here? Also mods, please don't move this to BBV, it's just this has not happened to any of my close friends and I have no idea how to deal with this. Thanks.