Should you get involved with a girl who has a boyfriend?

Unless you want a broken nose or black eyes, I wouldn't go there. Obviously she's going with him because she wants too. I doubt the code has changed in your generation as to what mine was: When you are dating someone the two of you are together and that's the way it is until when and if the couple split up.

I think you knew the answer to this question.

OOoh things have changed. IT'S CALLED SNATCHING