My brother agreed to take car of a person's dog the owner is not returning phone calls, is the dog considered abandoned

Doggy foster care: In June 2008 my brother agreed to take care of a dog (no compensation)while a woman took care of her sick husband, thinking it would be a few weeks. It doesn't look like the man will recover. The lady said several weeks ago that she didn't want to be bothered with the dog, but now won't return any phone calls. My brother doesn't really want another dog and will help find it a good home, but has no idea if the dog's shots are even current and not sure of what he should do. Is the dog considered abandoned? Should he write a registered letter to the woman, but what if she doesn't respond? - Is this your question? Add additional information