L.M.
I have been there and done that. I have been married to my husband for 6years and we have a "blended" family. My 2 and his 1. When I first got together with him he had custody. And his son started calling me Mommy almost from day one because his real mom had very little to do with him. Over the course of 2 years I watched as my husband allowed his ex to come and go as she pleased with their son. I had heard it had been like that all along. He only wanted to keep the peace and make his son happy. When she finally saw that her son had no interest in her, She started acting like a mom. And he (my stepson) started loving her and wanting to be with her. But, here's how we got it to where she quite coming and doing whatever whenever. I put my foot down and told my husband either you take your son's emotional health seriously or I'm gone. He laid the ground rules. You follow the visitation or you don't see him. These are the terms and I won't budge. If on occasion you want to see him when it isn't your turn then we will talk about it. My stepson benefited from this. He got to the point that he KNEW when it was time to visit mom. It gave him stability and he grew to love her so much more. I know that it is harsh and that it is drastic. But, my stepson still remembers that I was the one that was there for him the most when he needed me the most. He was 4 at that time and now is 10. To this day, I am still his Mommy. And I never wanted to take her place. He knows who is real Mommy is and in fact is now living with her.
As for the speaking bad of you, your husband needs to let her know that this is only damaging their child. He needs to tell her that it will not be tolerated any longer. I know that at first it will cause more problems. But, in the longer run it will settle down.