That's what I was saying before, Save. It may SEEM useful, but it really isn't. You can't trust that it's true or accurate, and whatever she tells you would be hearsay and inadmissible in court anyway.
Just shut her down when she brings him up, and tell her kindly that if she can't manage that you are going to have to end the visits. And then DO it. Next time she starts, just say, "Visiting time is over, will look forward to seeing you when you can respect my request not to discuss what your son is saying or doing."
Boundaries, m'dear.