What are his symptoms of sundowning? Does he exhibit agitation, anxiety, confusion, aggression? How would it benefit her to know what's going on? If she accepts the fact that he has dementia you can tell her that his body's clock isn't working right because of the disease. Doctors don't know what causes sundowning. If he gets aggressive during these times, I would contact his dr. If not, I don't see what trying to explain it would help.
Does your mom also have dementia? Is she aware that his brain is not working correctly anymore? And are they living at home alone? From your profile if they are at home your mom is going to need a lot of help as I would have my doubts that she would be able to care for someone that has had a recent amputation. How is she reacting when his Sundowning begins? It is possible that she may benefit from medication that could help reduce her anxiety. The other thing that might help is to reduce your dad's sundowning symptoms. Anti anxiety medications Turning on lights to reduce shadows and increase the visibility. Closing blinds and curtains to reduce reflection in windows. (this is going to become more of a problem in a few weeks when the clocks are turned back!)
Is she aware that his brain is not working correctly anymore?
And are they living at home alone?
From your profile if they are at home your mom is going to need a lot of help as I would have my doubts that she would be able to care for someone that has had a recent amputation.
How is she reacting when his Sundowning begins?
It is possible that she may benefit from medication that could help reduce her anxiety.
The other thing that might help is to reduce your dad's sundowning symptoms.
Anti anxiety medications
Turning on lights to reduce shadows and increase the visibility. Closing blinds and curtains to reduce reflection in windows.
(this is going to become more of a problem in a few weeks when the clocks are turned back!)