hi mother in law. your son's situation (and yours) sounds genuinely horrifying. i hope everything came to some resolution between them?
As long as you know, that's what really what matters. Maybe others here will have some proven strategies.

this is a major issue for me, and if others have strategies i'd like to hear them too. my w was not directly aggressive (except once), but passive-aggressive to an infuriating, juvenile, and on a few occasions physically endangering extent - and
of course the problem was my reaction, not her actions. i never told anyone about these things to some extent out of normal discretion, but also because nobody would believe the things i'd have to tell. so i said nothing, and then she left, i was just so awful. but it seems she had so effectively shaved the truth that someone opened the door for her, and the friends she discussed her plans with didn't impede her infidelity. now, i know what the situation was, including how i contributed (i really got frustrated and blew my top a few times). but knowing isn't helping me. so, strategies please.