My grandmother started saying inappropriate things when I was 13. She declined steadily and died when I was 22. I think she was 84. I know what this girl is going through. At least her grandmother sounds like she's in a place where she won't hurt herself or property. (Mine microwaved pots, put cereal bowls on the stove and 'watered' the livingroom with the hose). We had to ensure 24-hour supervision in her home.
My family really tried to keep her socialized as long as possible. Even when her vocabulary dwindled to "so bad" and "wonderful" we celebrated birthdays and Christmas with her. I do think her dementia made me lose some of who she was... I just couldn't remember her cooking or generosity after a while. There were times we had to physically try to hold her down or back because she wanted to leave the house and couldn't understand. We had to bolt the door. It was very hard. I admit I didn't visit as often as my parents, especially in the last year or so.. I didn't go to the viewing because I didn't want to see her like that and wanted to hold on to memories I did have.
I really sympathize with that teen on this one. I think she should still go visit, but perhaps not everytime. It's a wake up call to a lot of people that we may and probably will have to care for our own parents and the types of things we might have to deal with.