You are so right Slim....I'm dealing with a rescue (picked up 2.5 weeks ago) who scored about a 1 maybe a low, low 2 on the Henneke scale. After consulting with my vet, we used a 1/2 dose of wormer, with the other 1/2 scheduled in 1 month from the first. Then, in one more month, she will go onto the normal worming schedule with my other horses. She continued to look pretty rotten for just about a week after worming and now is beginning to pick up considerably.
I have heard stories of horses reacting very poorly to shampoos designed for other animals. You might try just a small area first to make sure you don't get an adverse reaction. But, something medicated for horses sure might be the way to go. I've seen some really thin, thin horses that haven't lost hair. I would be worried she might be packing a fungus or some type of crud like rain rot. Could your husband run a culture on it?? She'll certainly feel better after a bath and lots of loving. Bless you for saving her!!!