My gut instinct, and you really shouldn't trust it, would be to burn all bridges, and do your best to strongly dissuade any further correspondence.
But there's two reasons this maybe isn't the best idea. One, while I cannot know the vagaries of your situation, that relationship may be salvageable through better communication, but this requires willingness on both sides (sounds like they're not the type to concede though). Second, it is not always so easy to totally cut someone out of your life in this manner, and you'll have to accept, if any, repercussions down the line.
Console yourself with the notion that in my case, while I have a fairly good relationship with a foster parent, that person's ex-wife is an absolute monstrosity of a human being. After wresting control of the two kids they had together (as well as material assets), she attempted to completely deny them access to their father, and despite visitation rights, she coerced them into not taking advantage of it. One of them has only begun to speak to me now they've left her shadow.
So, tl;dr, everybody has their family shit to deal with, and it could always be much, much worse. No reason to exacerbate things, just set firm limits, and make it very clear that these are, in fact, limits, and not just another arbitrary rule that'll magically dissolve for them when they complain loud enough. She'll either learn to deal with it, or make herself look an absolute fool in the eyes of anyone reasonable.