Oh My God, that woman is a poor excuse for a human being.
How could anyone possibly have a dog for 15 years and just because it's getting old and all the health things that go along with that want to kill the dog because of that?
She'd better hope that the people around her care more for her than she did her dog when she gets old. Otherwise, she will get what she deserves.
I think you did the best you could under the circumstances.