You can coat the comb with Vaseline or other such to help minimize frostbite, the effected areas may heal or may just fall off which is most likely in my experience. It won't hurt the chicken's longterm health. The most important thing you can do is eliminate the frostbite from occurring in the first place. Frost bite is caused by dampness in the coop then the cold freezing that water, ensuring you have plenty of ventilation to rid your coop of moisture will likely stop future incidents.