As Cat said, I would play the neglect card for sure. If you have pictures of what the dog looked like when it first came to you that would help out a lot as well.

You have all the vet bills, and you have all the other expenditures (food, etc..) on paper too.

Any chance you have a written documents from the owner and yourself stating that she is transferring the dog over to you? That would most likely put the nail in her coffin.

In my experience in the Criminal Justice field; having evidence beyond reasonable doubt that a crime (animal neglect) occurred will be enough for you to keep your dog.

Keep us updated though, I'm curious to see how this plays out.