Hi
My sk's do this as well. In our case, I think it's because, like smums, they are not allowed to mention my name at their Mum's or talk about anything nice they have done with us but are actively encouraged to say "bad" things, such as if they have been told off or have argued with one of our other kids. If they tell "bad" things, they get a gift to cheer them up for the "awful experience" at our house. They tell us this, because they get upset that she then comes to us and says "why did you do x,y and z, they are so unhappy, they hate it there......blah blah blah" but they do it because they want the gift. If they ask to call DH, they get smacked 
So here they will say bad things about their Mum, she is a rubbish cook, she is ugly, they hate her....etc" where as I am the best cook in the world, pretty, kind...etc - DH has had to sit down with them and explain that the same doesn't apply here and that they can share whatever they want, good and bad and also that they can call or text her whenever they want. They are still wary but gradually we are breaking the negative cycle and getting them to share some of the positives about home as well.
I think all children will say what they think someone wants to hear, but I also think they can sometime suse it against them as well. My SD got annoyed with her Mum recently and yelled that she wished I was her Mum. She told me about it, and admitted she had said it because she knew it woud upset her Mum more than anything else, which I can understand.
I have no real advice - maybe just make light of it and point out her Mum's good points as well.