Someone explain to me how the fact that Haruko is so stupid she can't even recognize a person asking the same question twice is wholesome
There isn't enough information to say definitively whether Haruko knew what was going on or not. From
chapter 1431, we can see that she can be quite perceptive, so I am hesitant to believe she wasn't aware.
The wholesome part is that, despite the dementia, Haruko treated this person as a person, instead of as a problem that needs to be handed off to someone else, as Ishii-mama was so close to doing. This kind of behaviour is Haruko's ongoing theme seen in her other relationships with Yuki, Akane, and Tomoharu. Where they were each ostracised for having perceived problems, which Haruko ignored, befriending them anyway.
The part where Haruko is dumb, leading her to not understand the gratitude, is that she hasn't caught on that most people aren't able to do that. She probably has no idea that elder abuse is alarmingly common.