Couch it it terms of her.
'You are 18 and an adult, you want me to respect your privacy. You don't want me to breach that (give an example if you can think of one). That is your right as an adult.
I also have that right. I have the right to choose not to have someone who (give examples of what she has done) to be in my life and to know anything about me.
It is your responsibility to protect my right just as it is my responsibility to ensure your rights are protected.
It would be like a boyfriend of yours who became abusive. You have a right not to have him in your life, and as someone who loves and respects you, my responsibility to help with that.
So as an adult, you have aright to be in contact, but you don't have the right to give them details about me.
If she asks, just say 'mum has said she doesn't want us to discuss her. We will respect that. If you want to know the answer you need to contact her yourself. ' then repeat.'
This might help her understand. If your mum hasn't changed, then I would imagine she will up the pressure on your dd which will help her understand why you went nc in the first place.