Son's fiancee has forgiven him for cheating but she still is horrible to him?

My son is 24 and his fiance is 25 and they have been together 5 years and well they are supposed to be getting married in November. And well his fiance and him have a 5 year old son as well as a 2 year old son. And well i keep seeing my son with this other lady and and i have seen them kiss. I feel so torn about this and i am not sure what to do. I did not raise my son to cheat around on women and disrespect them like this expesliy his children's mother. I just know that this will hurt the kids more then anything else. And i also know they both have been having a hard time with there relationship and everything.And well i sat him down and talked to him about it and he said that his fiance has been throwing things and that she has been hitting him and that he can not take it from her anymore. And i suggested counseling and my son started to cry and told me that he suggested that to his fiance and she just blew up at him. I am not even sure were to go from here and now i do not feel so bad for his fiance if she is being abusive to my son. I am not sure what to do. And well anyways my son and his fiance want to work through it all he swears he never had with the other girl and his fiancee still loves him deeply. And she does not want to split the kids up and split there family up but she has postponed the wedding which i support. My son has been feeling really guilty to the point were he wont even touch his fiance and he wont kiss her he swears he love's her. And i believe he does but his fiancee and i are both at a lose. They are in marriage counseling but from what his fiancee has told me is they are not making much headway with him he is just clamping up and he just sobs. He told me that he is no longer cheating or anything like that and he feels terrible about cheating on his fiancee. And that he wants to stay with her and be there for his two boy's. His fiance and him have worked through most of it and they have decided to go ahead with the wedding. She says she has forgiven him but she still holds it over his head. She was partly to blame she was verbally abusive and still sometimes is. I feel like they are just headed back to this crossroad if they can not truly fix this i am unsure what to do. Advice?