I had a buddy do that. Hit on a girl that I knew, and were kinda flirting with. Regardless to say were not buddies any more, and how I didn't pummel the living crap out of him, I'll never know. I go for Chuck in this one.
This is what people differ in, in my opinion.
ALmost the same thing happened to me with my schoolmate, but i had no impact on us and we remained friends.
After all, it was the chicks choice, and I would always rather see girl happy with someone else then unhappy with me.