Now that I've read everything I was supposed to read in the first place, here's my revised comment:
880 lbs is thin and for her to gain what the vet wants her to gain is just about spot-on.
Regarding her hay: she needs to eat 2.5% - 3% of her body weight daily; the weight you WANT her to be.
Take a 30 gallon garbage bag and your bathroom scales to the barn. Stuff the bag full of hay until you get to the 2.5 - 3 lbs. Get a "mental" feel for that plus look at the bag to see how stuffed it is. That will give you some idea of what she needs because hay flakes can vary greatly in weight.
Be sure to fluff the flakes loose before putting the hay in the bag.
If you have a Tractor Supply close by, they should sell Manna Pro's rice bran. I feed equine pellted rice bran to all my horses. It is 22% fat but it's healthy fat and provides horses with a cool energy as opposed to the jittery high they get from certain other feeds.
If you try the rice bran (in addition to a ration balancer not by itself), be sure the bag says for horses and that it's stabilized and calcium fortified, which Manna Pro's "Maxi Glo" is.
Even though the alfalfa's good for weight gain, I wonder if she's a horse that reacts to alfalfa. My two 20-somethings both get well soaked timothy/alfalfa cubes and do very well on them.
The 12 yo and the 16+ yo don't get any because they get ugly, even on just a couple cubes. The 12 yr old is insulin resistant and even a couple good sized cubes will spike his insulin to where his cute little lip nuzzles turn into nips.
Hope this helps:)