You live and learn with dementia.
When we were very new to it, with my FiL, we blithely thought there must be an easy answer to things, like the same questions he was asking incessantly.
We actually made a laminated A4 sheet, so he could keep it with him, with very clear answers in nice big print, to the questions he kept asking (I once counted the same one 35 times in one hour).
Did that laminated sheet make the slightest difference?
It did not.
I was a lot more clued up when it came to my mother, several years later. She became obsessed with the idea that her sister had 'stolen' their mother's house.
Any efforts at logic/reason, lasted only a day or two. By that stage, even a signed and sealed assurance from the Lord Chancellor wouldn't have convinced her - he'd have been 'in league with' her sister.
I just started saying e.g., 'Dear me, that's awful - I had no idea. I'll get on to the police/a solicitor first thing tomorrow.'
Worked a treat. (Until the next time, maybe 10 mins later - rinse and repeat.)
Luckily she never saw that sister, so there were no personal attacks to deal with, thank goodness.