Timothy hay pellets or Alfalfa/Bermuda mixed hay pellets, 2 cups of milled/ground flax, a vitamin supplement, and a digestive aid (pro/pre-biotic w/yeast cultures). That will give her all the nutrition she needs, fat for weight gain, and help her system absorb it all. Hard keepers (IME) do better on NO grain or grain-products. You'll see better weight gain and you remove almost all risk of founder. Add water to the hay pellets to prevent choke. You can feed up to 5 lbs per-feeding of hay pellets. Alfalfa mixed pellets will be higher in calorie and protein, but you don't want to give a founder prone horse too much alfalfa.
Pasture can cause founder too, if the grass is too rich or has been stressed by drought or heat. She might need to be dry-lotted with free choice hay if the pasture grass is a problem.
A third cause of founder is mechanical. If the feet are left untrimmed or trimmed improperly a horse can founder. Keeping her feet in good condition will be very important. I would keep her on a 4 week trim schedule.