My paternal grandmother and maternal grandfather both had Alzheimer's.
I think, when someone first starts to develop it, some amount of helping them to remember what's real and what's a memory or reminding them of things—keeping notebooks and putting notes around the house, etc.—are good and necessary. At some point, though, it's going to progress to a stage where they aren't going to get better and any mitigation is possibly unhelpful. Unfortunately, you come to a place where keeping someone comfortable is often more important than keeping them healthy (or sane). Just to say, when they can no longer care for themselves and are constantly bouncing around in their own memories or completely oblivious to what's going on, reality ceases to be important.
When we would care for my grandmother, I would play along. For example, she would say things like, "Do you ever get married anymore?" And I would say, "Sometimes, but not as much as I used to." And she would nod, as if satisfied with that answer and change the subject... It was a lot easier for both of us to just have exchanges like those than for me to go into a long attempt at trying to get her to remember who I was and that I was only seventeen and had never been married.
My other grandmother, for some reason, was very honest with my grandfather (whom we only recently lost). My mother died some years ago, and when he asked for her, my grandmother would tell him she was dead. I felt like that confused and upset him. So when he asked me (he didn't remember from one day to the next what we told him about her), I would say that she wasn't there right now but he would probably see her soon.
There was a period where he didn't know me at all, but he thought I was my mother (his daughter). When that happened, I just assumed the role and pretended to be her. When he asked about my husband, I told him what Dad was up to. When he asked about my children, I told him what my sisters and I had been doing lately. When he got to where he really didn't know any of us and didn't seem to remember anything about his life (and was mostly not even communicating anymore), we just acted as if we were kindly strangers he'd met and told him pleasant things, but we didn't try to make him remember us.