Feed little & often, as horses digestive tract is designed for. At least 3-4 small meals per day, the more the better. I don't advise feeding sweetened or grainy feed, but if you do, it's even more vital you follow the little & often rule. She'll be getting very little from it only once per day & it can cause other problems fed infrequently &/or large meals.
I'd also be giving her some lucerne/alfalfa hay too, as it's likely she needs more protein. The 'baby' would also have sucked the nutrients out of her too, so it may be imbalanced nutrition rather than lack of calories that is making it hard for her to put weight on. A good quality complete supp should take care of that.