When is it time to give up?

A friend of mine is going thru a very difficult time with her ex. She really loves this person, but he keeps lying to her and messing around. They go back and forth to one another.  She just can't seem to stay away from him and just when she thinks he has changed and he won't do it anymore, there he goes again.  I feel terrible for her because she really loves him, but he always breaks her heart.  It is tearing her up inside. What is wrong with some men that they can't just love one person and be with just one person?  Don't they care about how much they are hurting the person they say they love?  How can one person be so ruthless and heartless to not even care about how much they are hurting that person who would do anything to just to have this person forever and have them be committed to them and only them. He doesn't even realize he is torturing and breaking her heart. When you make a promise to someone you should keep it.  Men talk about women not knowing what they want. Some men are worse then women. They can't even decide which woman they want. And if we did what they did, we would be a lying cheating bitch. At least when we make a promise, we keep it. They can't say the same. Tell me how can you hurt someone so badly, you claim to love?  Is the thrill of cheating really worth fucking up your life and someone else's life?  They need to remember it is someone else's life they are fucking up & have no right to do it.