The best thing to do is attack with kindness.
If you act like a jerk to the guy, your girlfriend, and her other friends will think you are an ass. They might even say something like "you should date someone better."
If you act nice to him, he may feel worse for moving on your girl, and all of her other friends can't say you are a jerk.
There are times when her friends will want her to break up with you, because they want their other friend to be happy.
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