It is wrong right? so what is the question? - do what is right!

Besides it being morally wrong. She is more than likely going to get hurt. Try to make her understand this.

If the married man is in a monogomouse relationship then you need to tell her.

as a best friend I think it's appropriate to make your feelings known but just once. If she's in love with this guy then there isn't much you can do but be there when the chips fall.

At the risk of sounding like a jerk, your friend deserves what she gets. The wife might get hurt due to your friends behavior.

How old do you need to be to know this is not good. Where is the benefit? Can she have him anytime, anywhere for anything? If he lies to his spouse then he is good at lying and your friend will not be the exception. Personally there are some things I am not willing to share. If I say about my boyfriend "He's okay.", my friend's state, " Okay is not good enough. You (your friend's name) deserves better."

Explain your view of the situation to your friend and leave it at that. If she is diluding herself enough to have a relationship with a married man and think its acceptable then she probably wont listen to your words of wisdom. The more you try to seperate them the more it will hurt your friendship in the end. Just be there for moral support and a shoulder to cry on when she realizes the mistake she has made.

Express your opinion to her and do nothing, is not your problem to get involve in!

A terrible situation for your friend. Just be there for her. She will learn over time what the real situation is, and she will listen to you and you can be there for her then. Feel bad for her. Many girls end up in this situation and I hate when people act like it's all them and they are evil mistresses. The truth is that the married man is usually older, and manipulates things so that it appears to the girl that he is only "technically married", that the marriage has been over for years, just haven't gone thru courts yet, that the wife is already cheating, doesn't love him, won't have sex with him for years, etc... The truth is that he is probably happy with his wife and after he has sex with your poor friend he goes home and f#cks his wife. He will tell your friend he loves her, and has every intention of being with her, he just has to take care of his home-life situation first. Problem comes when it's been months, if not years, and he never takes care of it. He only sees your friend when HE can. He is unavailable. Your friend will start to feel very alone. Then she will start to feel used. Then finally she will question what his intentions are towards her, and that is when you can step in and point her in the right direction. Hope this helped.

In situations like this it will be hard to get the point across that she should not be in this situation, simply because she will not listen as she will be clouded by her feelings for this man. The only thing you can do is as a friend talk to her on a level, if she doesn't want to listen then you need to leave it at that...but be there for her if/when she does fall.

What her because she might be with your man next.