How can we prepare my father, who has early Alzheimer's for the news that his wife is terminally ill?

My question is partly a response to Paula Spencer's article about discussing sad events with a person who has dementia. This article gave good suggestions when remembering a sad event. I am interested in any advice when the event has not yet occurred but is inevitable. Both of my in-laws live with my partner and I. Her father has Alzheimer's and is still (I believe) in the earlier stage. Her mother has leukemia and we were recently told that she has between 1 and 6 months left. We are working on getting my father-in-law used to being away from her - he started "volunteering" at Adult Day Care this week. He seems to be mostly aware that she is ill, but he seems to believes she will either get better, or not get worse. How, and at what point, should we try to let him know that his wife is dying? Michele