Yeowch! You and your farrier have got your work cut out for you- I am glad your farrier is the kind that is willing to seek help or refer as necessary. That's always a good sign to me that they're willing to learn and probably already ahead of the curve anyway.
Your girl doesn't need a supplement, per se- she needs a complete nutrition plan that will help support her and grow some healthy feet. I would start off feeding her like an insulin resistant / sugar sensitive horse since if she is at all reactive that way it will hurt her feet in the long run. Essentially, go for lots of forage and fat, low sugar/starch, and avoid grains and other high sugar feeds like stressed/rich grass pasture, corn, etc. Then add in some good vitamin E, omega 3s and biotin, which are all in something like horseshoers secret or other high quality coat and hoof supplements.