Friend went snowboarding in sub zero temp, got blistering and swelling on top behind his ear. Head was completely covered. Frostbite? Irritation?

Cold injury. Depending upon the time of exposure can lead to tissue damage from freezing injury to the back of the ears.

Possible. Frostbite is possible even if he felt it was covered, when you perspire with exertion, the moisture makes you more susceptible to any area exposed. It could also be an allergic reaction to the material used to cover his head, or even a shingles outbreak. It would be best to have a doctor check it out.