That is really frustrating but I am not sure what else you can do.

Talking to her doesn't change things. It seems she is lonely and using you for emotional support and/or venting so her troubles don't prey on her mind. And no, she is the parent and should not heap her marital issues onto you. No parent should do so. There is a clear blurring of boundaries that she doesn't seem to be aware of.

Maybe if she had friends she would be less inclined to do this. Seemingly, you are the only one there for her as a source of emotional support.