Seriously, if that happened to me, I'd call the guy and tell him off......and I definitely wouldn't see them again. I would also tell my friend, and hope they could understand that I had no idea who I was sleeping with. If it was a friend of mine, I would feel compelled to tell them their partner was a cheating scumbag so they could be rid of them, even at the cost of our friendship.

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Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi