I think it all depends on the situation though. I was kinda in that boat now with a girl I was seeing. After a few years of being married, her husband treats her like shit, never wants to do anything with her, and really could care less about her. If you come along and treat her how she should be treated and keep up with it, I don't think there'd be a problem. People get mixed up in shitty relationships, if you save them from it, I don't think she'd leave you that easily.
HOWEVER, that's in this situation, if her husband is a great guy and she wants to leave him for you, then watch out
ha ha.