I know it seems counter intuitive, but moisture is the cause of frostbite, not cold. We have had Leghorn roosters with huge combs who spend the night outside, in a hedge, in -20 F (-29 C) winter weather. They did not get frostbite.
Once we had them spend the night in the chickenhouse, where we used heat lamps and heaters to keep them warmer, they got frost bite.
After doing some research, we learned that moisture is the problem, not temperature.
Vaseline will help, but it's not the best solution, as it is absorbed through the comb into the body...