Q: My father, who has Alzheimer's, keeps asking about his mother, who died many years ago. When I tell him that, he bursts into fits of uncontrollable crying. How should I handle it?
A: Enter you father's reality wherever he may be. (Read: "Getting into a Dementia Patient's Head")
What your father is thinking and feeling is his reality. He is living in the past, where his mother is alive. Many Alzheimer's patients live in the past, and it's easier for the caregiver to go there. There are times when you can try to explain true reality, but chose your battles.
Other times, it may be more productive, and less stressful, to enter his reality. If he asks about his mother and he thinks she is still alive, then talk with him about his mother from a positive perspective. Ask him about the favorite dish she cooked for him and other stories that you remember he told you as you were growing up. If he asks where she is, tell him she is out of the house grocery shopping or something along these lines.