If it makes any difference, we can only deny for so long, and if her son is truly headed for trouble, sadly her time will come.
I can only speak for myself but I can no longer associate with drama and meddling. I excuse myself quickly these days if a conversation begins to go there.
As far as your friendship, I have a feeling that NOT telling her how this affects you will have a more damaging effect than telling her.
Maybe you could mention to her (and remind her when she gets on a roll) that you are sorting through a lot right now and would prefer to talk about something else. Be prepared though, it sounds like she may have little to talk about besides her kids and their lives.
Good luck
(((Hugs)))