Oh, honey. I do understand. I'm sorry I'm so blunt.
She sounds AWFUL. I would tear my hair out watching that...except that
she controls access to the grandbaby.So I'll be blunt again (since that's how I show you I care):
I told him it makes me sick of my stomach each time she gets in a rampage of ugliness. He told me, go vomit to your house and come back when you feel better.
This is where your answer is. You feel what you feel (and boy would I feel the same!) BUT because of the impulse control AND boundary-with-adult-child problems (you know I know this because I FAILED at it, and my D no longer speaks to me)... you say it to him anyway.
And he, clearly, very clearly, has told you exactly what he wants you to do.
He wants you to NOT comment. So that's why I vote No Comment (from you, about her--evermore).
At the same time, another thing occurs to me... if you are distressed and upset and destabilized by being around her behavior, well there's an opportunity to set healthy boundaries for YOURSELF. You have no obligation to sit there passively and quiescent while you observe abusive behavior.
But rather than taking it up with your SON (which distresses him, obviously) -- you can just speak about YOURSELF. Your OWN well being. For example (and take this with a grain of salt because I've had a beer):
I love you all so much. But I'm going to have to take a break (go outside, not be available this week), because when you ARGUE/BICKER/CRITICIZE EACH OTHER I find I feel so upset that I can't enjoy being with my grandbaby. Please call me next week...I would love to babysit. But I just need a break from this tension. I can help and support you two but not when it's affecting my happiness. Please let me know when thing are calm and you can talk to each other respectfully.Repetez
ad nauseum.
See? That's talking about YOUR feelings and wellbeing. I think if you say how it affects YOU, not your son whom you cannot speak for, it could become an adult non-victim-triangle kind of dialogue.
[
Edit: Edited out a guilt-trippy little scolding that I didn't like when I read it this morning! Sheesh, ewww.]
Love you and I've MISSED you. Spent a lot of time thinking about your life and so so so wanting you to find happiness.
love to you
Hops