This is so individual. I do believe they need to be told when a death occurs.
However, the choice to remind him or tell him a believable story to distract him is up to the family. If it causes him so much pain, is retelling the "truth" each time, so he has to relive her death, right? Some people would say yes.
I had no choice with my mother. She didn't have Alzheimer's, but she did have dementia, and shared a room with my dad for a few months (they'd had individual rooms before that). But she was alert enough that we had to tell her, so we had to go through the grief over and over, but she did finally get it and remember it. With your dad, it may be different. If he is in a stage where he never will remember, maybe just saying she is "with Aunt Amy" now, and let it go is the way to go. If Aunt Amy died, he may not remember that, either, and may be comfortable.
My heart goes out to you. So many of us have to make these hard decisions. Sometimes the "right" answer is different each day. I wish I could offer you a miracle answer, but I can't. Just don't feel guilty if you find that such an answer as "she's out with...." works for him. What works, works. You are after the best quality of life he can have.
Blessings,
Carol
I cared for my Aunt for two years in the late stages of this awful illness.
In my opion there was no sense in making her relive the pain of her mothers passing every time she asked where her mother was. I just told her she was on vaction. This was much more comforting since she would forget that as well, but she did not suffer the pain and relive the loss. She asked several times a day and be content to know she was fine and would see her soon. In her final days, she was happy to know she was going on vacation with her mother and let go peacefully