This is so sad- but your mum has the right to feel this way. This illustrates so well just how vitally important it is to talk- when all is well, and tell each other how we see the future beyond the limit we set outselves. My mum was always clear she did not want her life to be prolongued in any way once she got past a certain stage. She saw no point in continuing to fight in pain and increasing loss of dignity- once it was clear she would not, and could not get better. I respected this, 100%.
And I have told my OH, and my AC, that I will probably feel just the same, if ever, like her, I lose my legs, my sight, my dignity. I hope I never do- and I am a very positive person and a real fighter- but I am 70. My mum died aged 94, but she truly hated the last 5-8 years of her life. I wish she had been given the gift of an earlier death. One of the reasons I ma choosing to live where I live, is to have that choice, always.
Take care.