It's good that she's eating and drinking well, though not wanting to stand at all is concerning. It may just be that she is so weak that standing is difficult for her. Is he peeing and pooping? I would worry about scald if she's not standing to do that as it will get on her skin. The fact that she has calluses indicates that she's probably been spending a lot of time down anyway, so I wouldn't worry quite as much as I would if it was a normal, healthy horse who suddenly wasn't wanting to stand. At some point, as much as you don't want to be rough with a horse who's had a tough go of it, she has GOT to get up, for her own good, and you might see if you can motivate her to do so... either that or she should be turned from side to side every 2-4 hours around the clock. :(
If you can get her through this, she probably won't look outwardly better for a month or more, but rest assured her body is putting the calories to use inside, fixing her organs and immune system and balance. Once that's done, in a few months, she'll actually have everything inside fixed up enough to start growing on the outside where we can see it. Horses grow up to 4 and 5 years old- she may not ever be as tall as she might have been, but she's got plenty of time to put on size yet anyway.
I wouldn't feed anything but hay without double checking with your vet given that she's a 'down' horse at this point. But hay, hay and lots of water. She'll be in my thoughts. I don't think I'm anywhere near you in TX or I'd come out and lend a few day's labor to her cause.