Remember that, yah, she may be a slut, but your boyfriend is more guilty that she is. He's the one who broke a commitment to you. So I hope you have words for him.
I guess what I'd say if it were me would be something along the lines of "What? Something wrong with you that you can't find your own boyfriend? Or are you just to immature to have one?"
Or, "May the fleas of a thousand camels infest your arm pits." Nothing gets the point across that you are unhappy with someone when you curse them to have to invest in flea shampoo.
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