Since she is so thin, I'd make very sure she doesn't over-eat just on pasture. Most pasture grasses don't have the required nutrients, to build up a mare in this condition. We have built up several rescue horses, who were very emaciated. We pretty much follow these guidelines...
Horse Rescue Part 4 – Rehabilitation of the Starved Horse | Habitat for Horses
Read through it carefully. It contains a lot of good advice.
In all our years of rescue, we only lost one. She was an old movie mare, who came to us in very sad condition. In spite of several weeks of intensive care, my daughter decided it was best to allow her to go. She came in with another discarded movie horse. Both were in their 30's, after what appeared to be a lifetime of pretty hard use. The other, is a huge black Thoroughbred, who is, three years later, still going strong, looks wonderful for his age and is a happy, healthy boy and dreadfully spoiled in his new home.