I have a friend who says "If I can steal her from her boyfriend that means he never really had her anyways"

I don't disagree, but I wouldn't go after someone with a boyfriend. (Yeah I say that now, even though I've done it on more than one occasion...) I guess it's kinda of a different situation if she's showing a lot of interest in you... the only 2 times I went after a girl with a boyfriend she showed interest in me before and during the boyfriend.

I had a friend who went after an ex-girlfriend of mine and it pissed me off... the same friend a year or so later had a girlfriend that was all over me trying to get with me, saying she'd leave him for me, etc but I never acted on it.