I would love to know how the fiance is negatively affecting the child and how that's going to magically change overnight once Mom does get married.

I understand the desire to keep new partners away from the children while the relationship is in its early stages but there comes a point where it's more about "How can I get back at Other Parent" than "Is this healthy for my child?".

Is that what's happening here? I have no idea. It's simply a statement of my opinion.