"She's the only one on Earth, besides him, who can decide if it was a disservice. You're not in the position to make that call"
You don't or can't know that. She's elderly too which means the likelihood of her suffering her own health catastrophe grows ever more likely & if that happens the family now has TWO sets of problems, one of which they could have been preparing for years.
I've seen both scenarios, the good thing is that the family with an otherwise healthy grandmother who began to slide into dementia, didn't keep it a secret.
The close family & friends were all made aware - so that one lovely summer day when she wandered off unobserved & we were frantically running up and down every street, one of our buddies had already found her TWO MILES away.
Since he knew her condition, he didn't just smile & wave as he would have done if we'd kept the secret - he grabbed her, put her on the back of his motorbike, stuck his helmet on her head and brought her back, giggling & smiling like she was the teen she no doubt imagined herself to be.