my dad always said we are more "humane" with animals than humans when it comes to things like these. Once an animal is suffering we put them to sleep, with humans we extend the misery to the max. I mean if there is hope one wants to hold on to it, but some times we are just prolonging their pain for our own gain. I completely understand what you are saying.
My uncle made the decision himself after so much chemo and all that it entails to stop treatment. he told his wife he had enough and to just allow nature to take its course. I can only imagine the pain he was going through.
Its ok and perfectly natural to want them to be at peace and done with the pain. As much as I believe in treatments some times I feel it is better to just go naturally... at least you can enjoy the time left.
So sorry your friend is dealing with all of this, and everyone else around her