I need some help..

So long story short I have a best friend who has a son, he is now 9 year old? She been dating some guy on and off for like about five years. This guy always cheats on her and always ends up leaving her for a other female. Any who the longest they ever stay was when this guy had one of ex pregnant yes by him. Than couple months down the road left my friend again. Now again his baby momma is pregnant and she with him again? I don't seem to understand why? My best friend son doesn't have a dad. But at the same time she keep having this guy come in and out his life? What would you tell her to help her understand it isn't right?