My mom (88) has informed my dad (86) that after 4 pm he is not allowed water. Advice?

Hydrate!
On the opposite side of the scale is what Ferris mentioned:
W a t e r. I n t o x i c a t i o n .
Look it up.

Helpfulone, this is tough on both of them. Two people with dementia trying to manage on their own is not a good think. If you were to move them to assisted living or to an Adult Family Care Home others could take on the caregiving role and then they could just worry about living day to day. It sounds like mom as always been controlling and that will only get worse not better as time go on. In the mean time while you are thinking about a move may I suggest guards for men. He can wear them inside his regular undies and change them if they get wet. This way she may never even know if he has an accident. They are more discreet then full on depends so he may welcome them easier. Taking his water away at night is not the answer. It's ok to limit it a bit but everyone should be able to quench a dry pallet when they need to, and you don't want him becoming dehydrated which can happen quickly in older folks, because this will bring on another whole host of problems. I wish you the best. Hugs to all.