Need major help, fiance wants to go back with her baby's daddy for her baby's sake even if shes not happy...?

My fiance has a 2yr old child and the father will not allow his child to be around me. She's taken him around me a few times and he even called me dada the last time I saw him. The reason they broke up is because he doesn't pay attention to her. He's a great provider to the extent of serving breakfast in bed before leaving to go gamble. They aren't sexually active and she says that all he ever wants is for himself. They rarely kiss and she got tired of everything and left. Now she's having second thoughts of going back because she says it for her 2yr old's happiness. It took her 6 months of trying to leave before actually doing it. She feels like she owes it to him and that he is the way he is because she cheated on him before. With me, I can genuinely say she does love me. She said she already knows that she will crave the affection we have and find me because they don't have that chemistry. She says I know I'm being stupid for thinking about this but I've been selfish all my life and I just want my baby boy to be happy. Problem is she stays at my house and her baby is staying at her moms. She seeing him daily for 3-8 hours but she can't sleep with him and it makes her sad. She told her kids dad that because he won't allow him to go with anyone, she's hurting. She asked me to help her get her own place so at least she can sleep with her baby. She also has a 16 year old with another guy and she regrets she missed her childhood because they're not that close and she wants to prevent that happening with this one. Now for us, she wants to stay together and she says all the time that I'm the one for her. We've done things she's never done before and she can be herself. She says she's very happy when were together and she feels pretty again cause he just puts her down. So what do I do? I'm here asking cause I'm looking for answers that I may show her to help her. I'd be okay if he didn't disrespect her and even told me to keep his kid.