If you don't want her to lose more of her comb, the first thing you need to do is prevent further frostbite. Put a heat lamp over her, find a way to heat the coop, or bring her into a warm house/garage. Put some petroleum jelly/Vaseline or other oil on the comb to help seal it from cold drafts. Make sure the litter in the coop is dry, and that there is enough ventilation, as moisture is a major cause of frostbite.
As for the frostbite itself, warming her up should help a bit. Try massaging the area to stimulate blood flow, and perhaps apply something like VetRX or Vick's Vapor Rub to stimulate more circulation. You could also put some antibiotic ointment, like Neosporin, on the comb to help prevent infection. Any severely damaged parts of the comb will die and slough off, but the less severely frostbitten areas will likely scab up a bit, but not be lost entirely.