Read other letters about this article
The locus classicus on this range of issues is Tolstoy's Death of Ivan Ilich. It is heartbreaking.
The effort to shield a loved one from knowledge of his or her illness entails many costs, both for the patient and for those around him or her. As Tolstoy reminds us, it is foolish to believe that the patient does not have internal thoughts and struggles about what is happening to him or her. A primary casualty of keeping secrets (at least in most Western cultures) is loss of trust.
These issues take a peculiar form in Alzheimer's (and perhaps some parallel conditions). What is told, with truth and compassion, on one day, may not survive until the next. So the decision of whether, and what, to tell is not a one time thing. At a certain point in the progression of the illness, the issue may come to have more to do with the ability of family and friends to be comfortable within themselves, and no longer with the understanding (to the extent that concept remains meaningful) of the patient.
I've though about this as a bioethics scholar, and experienced it as a child. There are no happy answers.