I'm a bit late here, but one of the early symptoms of dementia starting to take a turn might be it's harder for the person to reason with others. Reason why I think it might be is I remember back around Christmas 2017 I think, my mom wanted some candies made (can't remember the kind but I think it had chocolate and coconut in it), and sent my grandma a website that had the recipe, that website redirected her to malware and nearly caught her before she called her brother.
Said candy has about a thousand websites that have the recipe, but as hard as I tried to reason, she did not budge off of not making the candy because of that experience on one website. I ended up making a tres leches cake with her instead and my other grandma (that's still here now) ended up making the candy.
Now take that kind of situation and apply that to much tougher decision making with someone that has a harder time reasoning. It doesn't look great, does it?