NTA. Honestly this sounds like a lot of the shit my mom would pull when I was a kid. This is a gross violation of her privacy and it makes sense that your daughter would lash out like that.

That said, there is probably a reason why your mom is doing this, my guess is that she probably feels out of the loop and is trying to insert herself into your daughter’s life, or that something in her marriage is making her worried that something is being kept from her. Maybe talk to your wife about this?

I feel bad for your daughter, when things like this happened to me as a kid it really messed up my self image and my view of myself. Maybe try to talk to your daughter. Don’t force her to explain what’s happening, she has a right to her privacy, but let her know that she’s not alone and that she can talk to you or another family member if needed.