Could it be that the issue here is more about "Mother in Law" control than about the religion per-se?
My whole family broke away from JW in the early '80s. I just quit doing org religion altogether. My sister joined a bible-thumping fundie church. I do not think she has any idea about their doctrines. For her, it is a social club and a source of potential students for her dancing studio - IMHO. She makes a big deal out of dragging my mom there every Sunday - (mom has some dementia now, so the roles are getting reversed)
The incredible thing is that the mother-daughter power play and infighting just continued right on, JW religion or no, with the only modification being that now it is daughter controlling mom rather than the reverse. And the new holy roller church in the round rather than kingdum hall...
If that is the case with this family, then not only the religion thing, but also the control freaking must be dealt with. If I were him, I would see a lawyer.
Just a thought, James