Dear Nick, again peace to you and, sorry, but uh-uh, no... I ain't buyin' it. Perhaps if future MIL came on strong with the magazines... or kept pushing for a bible study... perhaps. Or if she took issue with the young lady's dress or appearance. Perhaps. But because she assigned them separate rooms? Since when is that an exclusive JW "thing"? All kinds of folks out there still adhere to that one, dear one.
My advice? Tell dear son that he really needs to get to the bottom of the problem (if he doesn't already know what it is).
Whether it's former GF... or dear son... sounds like someone's making JW mom the scapegoat. Sorry, but based on what you've posted that is my take. Of course, there may be more (much more) to it.
Again, peace to you!
A slave of Christ,
SA