She's not a prisoner, because there is no "she" just a mass of living tissue.
No cognitive functions, no memory functions.
So really, there is no poor kid and never was.
There is no "want" or decision, or knowledge, at all.
Living, it's no different than a plant. Life? Yeah. It's life. It's living, but this is without any human logic, by nature.
I wouldn't have hesitated pulling the plug from the get-go.
The way I see it is that the genetic structure was supposed to have life sustained with a human brain, which is completely compromised, so the quality of life is completely diminished.
There is absolutely no way of knowing if the life sustained is in pain. I doubt it, but I also don't think the stress on her family is worth the trouble, nor does it create any ethical boundaries to remove sustaining and continuing artificially.
The only reason she is alive is because others interfered with her death.