Do you have photos?
Is it blistered or healed over?
If it is healed over and you want to put something on it you can apply some plain neopsorin or vetericyn spray. I have found that applying something like neosporin to combs/wattles is easier if you use a q-tip - takes a little more time, but you can target the area you want easier.
Most of the time, depending on how much damage the frostbite caused, it will heal on it's own, but you want to monitor it to make sure there is no signs of infection, pus, oozing, etc.
http://www.the-chicken-chick.com/2013/12/frostbit-in-backyard-chickens-causes.html
http://www.fresheggsdaily.com/2014/01/treating-and-preventing-frostbite-in.html