Personally, if she were in my flock I'd cull her and chalk it up to a lesson learned: why it's important to keep a closed flock. Such an outward appearance that points to very poor flock management would lead you to wonder what internal issues you will also be seeing in these birds. A bird that has both feet so severely compromised by frost bite has been in what I can only assume are at best lack of ventilation in an overcrowded coop, and at worst, horrid management. Given the fact that she is underweight, there may be other disease factors involved.