That sucks. You could go with a mobile device management plan, like simpleMDM. https://simplemdm.com/features/
I'll think about other options this weekend and talk to my business team on Monday. There might be something we aren't thinking of that she wouldn't see. I know a few of the trackers can hide the app altogether as well, but they are pricey.
Or a family share Apple ID account so you could look in find my iPhone. Just create him a new Apple ID and link it to yours, log him out of the old and into the new, and don't tell her. She might not realize.
She sounds like a terrible person, btw.