Trotting, walking up hills and walking back down, w/t/c over ground poles really helped my horse gain some muscle
lots and lots of trotting circles. I did this with my mare who sat in a pasture for about 3 months and she's gained most muscle back
To help her gaining weight, I suggest you to do a lot of rise out with hill work so she works the whole body withtout the pressure of the arena. In addition to regular and progressive work, you should give her food with a lot of protein.
Good luck :)
I would start by talking to your vet to make a plan on how to get the muscle back. You should exercise her as much as possible (preferably every day) working different parts,every time. A good exercise is walk, trot, cantering over ground poles - it really works the hind end. Just keep a consistent excercise schedule and be patient because over time she will gain a lot of muscle back. Good Luck!!
Definitely would go on a good suplement! Give your vet a call and also another tip is stretching your horse. Also please subscribe to my YouTibe channel @dev.handy