Yeah, based on the story it's possible he does need to not be living alone - for example if he's having the kind of memory problems where he truly needs to be given a phone call to tell him to go get dinner now or he won't be able to eat, then he probably needs to be in some kind of assisted living, and that's the kind of thing can that be really well hidden until somebody is suddenly widowed. (It could even excuse the being horrible to the employee, tbh, though it sure doesn't excuse SIL - short-term memory issues can cause huge social problems and trying to mask them ineptly can make it even worse.)

But LW doesn't seem to think that's the situation; if it really is that bad, to the point he can't be medically trusted to do something like turn on the stove to heat up soup twice a day, that's a much worse problem that LW really should have confronted before writing this letter.