Life is meaningless. Eat Arby's.
LOL Arby's
But seriously, it's not a question of being married or not, it's the fact that she's in a relationship, you have to be a total dick to do that to the woman and in turn to the guy who has no idea what their woman is like, the decent thing to be done would be to give the guy a fair warning so he has a chance to dump the bitch if she's the one initiating it.
But wait, I forgot, that would be doing the right thing and humans have a hard time doing that and will find ways to justify their crappy treatment of other people won't they?