What should I do if a guy with a girlfriend is flirting with me?

This guy flirts with me but he has a girlfriend. I know he flirts with me, that's not the confirmation I came here for. All my friends and some of his friends see it. I just don't know what to do about his girlfriend. I want to tell her, but I don't know how. Shes an aggressive "fighter" type, and I don't WANT to fight with her.

I'd feel bad if was the reason they broke up. I don't know how to tell him, since knowing him, he'd brush it off. He's a nice guy, I hope to still be friends at the end of all this! :( Leave advice/opinions?

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