What would cause her constant pain? She is not dying of a terminal illness. I cannot imagine advocating starving and dehydrating a woman because she may feel some pain and there is no written directive. Do an internet search on the effects of dehydration on a human body. It is not pretty.
A traumatic brain injury. Certainly that has to be much worse than an headache. Clenched extremities. Catheters in places you don't want to think about.
What would cause pain?
A traumatic brain injury. Certainly at least a headache.
Perpetually clenched limbs.
Catheters in places you don't want to think about. (Hint, she cannot use the bathroom.)
So I'm just looking for an answer to a basic question, you are ok with her living a life of constant pain? Or you don't care if she's in pain or not, beyond the situation at hand.