I wonder is it just that driverann and her daughter have different standards of tidiness from SiL and he doesn’t actually have a mental health problem. If anyone came into our house just now they could describe it as a ‘tip’. Neither me nor DH have ever been tidy but he is worse! If he has a woodwork project he does it in kitchen as shed is too full of stuff to get into. He also uses kitchen to transplant seedlings so sometimes have compost on worktops.
Our dining table at the moment has a box of nails, some scrap wood, a box of wine-making accessories, my candle-decorating project as well as lots of other stuff which doesn’t belong on table.
One of our sons was quite tidy and now has partner who also doesn’t like clutter. They were here for a few days once when we were away and did a big clean-up, but it didn’t last!
If my husband is away, I try to de-clutter a bit, as he certainly doesn’t like me trying to organize his tools. But I’m nearly as bad with my stuff.
Not sure if any of this info is helpful to OP. As others have said, it’s up to daughter to sort out her husband and house. If my mother had come and tried to sort out our house, apart from helping with washing up etc, I’m sure it would have caused friction.