Detach with love. Simply make a boundary that you will not discuss her son with her (which means you don't talk to her about HER choices with him either) and then enforce it.
If she begins talking about him...walk out of the room, hang up quietly, but just remove yourself from the conversation.
Let her call you...and hang up the moment she brings up his name or raises her voice to you.
I've done this more than once and I promise you that it gets better the more you enforce it.
You don't have to be a willing participant in her toxic conversations. You don't get to tell her how to live her life with her son....but you get to live your life however you want.
That's called "recovery".
Hugs