That is a good (and frustrating) question, Sunsetgirl. I assume the door has a peephole. If it doesn't, that would be good for your mom, and they aren't horribly expensive to have installed.
What I would look into is a lock, perhaps low on the door,that your dad wouldn't think to open. Or even more than one, if necessary. They don't have to be too hard for your mom, physically, just tricky enough so your dad may not think to open them. It would at least buy your mom some time.
More extreme would be a lock placed on the outside of the door, but then your mom would have to go out another door to open it, and it could be a danger if they had to leave the house quickly, say because of a fire.
I hope you get some other suggestions. I'll be watching for them. Everyone has different experiences.
Remember to take care of yourself. You've got a long journey ahead, and it's a good habit to start early.
Carol