I have to say... of all the rescue stories I've seen, this has got to be the worst (conditions).
As everyone has said, TAKE THE SNAKE. IMMEDIATELY.Even if you are used to caring for dogs, you CLEARLY care about the health and well-being of this poor girl. You will do wonderfully, and you have come to the right place. Kudos to you for sticking up for her and giving them where-what-for, but I do think you should take the snake.
Please keep us updated, and search for some more threads on rescues. You will want to feed, but not too much too fast. Start off quite slow, but consistent. It will take a lot of time and patience - and probably money as well, if there are other health issues going on, which is possible.
I truly wish you all the best. Take care!