DH was too giving to his ex (they had a son together who was 5 when we met) when he and I first started seeing each other, but it came more from a place of guilt at leaving his child - he recognised the relationship between him and his ex was over a long time before it happened, but leaving his son was harder and he over compensated in some respects. Didn't help that ex lived in the (jointly) owned home, and had never been on her own in her adult life - she would call for favours, repairs, extra money etc.
There weren't too many clear boundaries to start off with, so DH would see his son and stay in the house for a few hours, having dinner and cups of tea. He stopped doing that after about 8/9 months as it was having a negative impact on his son who got confused and upset. Ex started seeing someone new and to her credit became far more independent and didn't need DH as much.
Now, 7 years on, they get on well, and occasionally text about their son (12, so he has his own mobile for contact) and other sort of relative things. They can have a joke, no animosity, and do things jointly like parents evening when required. They wouldn't see each other socially or invite each other out for a 'family' day - that sort of thing did happen in the early days of the split but not since DSS was about 6.
I get on really well with her.