this is something from what I observed on someone.

There's a good friend of mine. She's in love. The problem is, she's in love with someone else's husband. 
I've know her good enough to know that this is not what she wants. It happened beyond her control. 
Before this, she hated guys that are married and still act like they're single. I mean, she really hate them. Then there was a guy approached her and tell her that he loves her. She was furious and decided to play around with this guy. She played along, until she realized she's starting to fall for him too. She quickly try to stop everything. But then, it's too late, I guess.
I don't know how good this guy is with words, but, if she's really into him; he's good. very good.
He's married. He got what people so-called 'experience'
He told her that he still loves his wife, but things just got out of control.
He fell for her. 
and from what I observed, being in love with someone that is married, is not fun at all. I mean, she can only contact him when he's available; meaning when he's at work, or away from his family. There were times when she really need him, and he's not there for her. What's the point of being her boyfriend if you can't be there for her, at any times?
I doubt that this guy really loves her. Even though he did tell her numerous times he wants her to be his wife.
when they got married, would things be better?
I bet it wouldn't. In fact, it will get worse.
I tried to talk to her, approached her as gentle as I can. I don't want to be like others that opposed her strongly. She's a really good friend of mine. I don't want her to go through all this, alone. and I'm still trying. She's trying to end this too. I guess she's not trying hard enough.
I don't know why but I understand her. 
Hope she'll fine someone else that is WAY better than him.