This is very sad to read.
While personally, I would not want to live with advance dementia. There are things that can be done to support some quality of life even in this stage. It's just so important to continue to remember and connect with people as people on the level they still can.
Namaste care is something well worth looking at. It can bring comfort and some pleasure even in very advanced dementia.
Teepa Snow's gems model of dementia and her videos on dementia care too.
Here is an example of how she describes the experience of moderate / later stage dementia:
Amber ~ Caught in a Moment
Caution Required
Like a particle trapped in amber, I am caught in a moment of time. It may surprise you to see how I take in the world around me.
I may not know you or see you as a whole person. I react to you based on how you look, sound, move, smell, and respond to me.
I like to do simple tasks over and over and may need to repeatedly move and touch, smell, taste, take or tear items apart.
While it may exhaust or frustrate you, it soothes me. I don’t recognize danger; you will have to safeguard my environment.
I’m intolerant to discomfort because my mouth, hands, feet, and genitalia are highly sensitive due to changes in my nervous system.
Therefore, activities like eating, taking medication, mouth care, bathing, dressing, or toileting may distress me.
Please notice my reaction and stop if I am resisting. I can’t help myself and one or both of us may get hurt emotionally and/or physically. If this happens, wait a few minutes, connect with me, and try a different approach; possibly substituting one area of focus for another.