Being with other men's women is unbecoming of a man, something undisciplined, irresponsible little boys do.
Don't hurt someone you've never met for some stupid slut, when there's plenty more out there.
And yes, I've done it - when I was a teenager - would not do it as an adult (though it has happened on accident, where I didn't know).
If you still want her simply tell her to break up with him first, which she if she was worth anything would've already done.
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