The recommendation is that if you CANT rewarm the part, then to leave it frozen. The thawing process will be excrutiating, and once thawed, infection spreads more rapidly than when frozen. Cover the part to insure the freeze doesn't extend further and further, but dont' try to thaw it by rubbing (you'll just do tissue damage) without a fire it will take far more heat than your body generates to re-heat a frozen body part, and you need to ADD heat to the part, not simply keep it warm(ish).
Frostbite usually affects an exposed extremity. In case of
frostbite, remove wet clothing and make sure the victim does
not develop hypothermia. Get the victim to a medical facility
as rapidly as possible. Do not try to rewarm the frostbite if
there is any chance that it might refreeze or if you are close
to a medical facility. If you are in a remote area far from a
medical facility, you may slowly rewarm the frostbite using
warm water (100°F to 105°F)
To the question of wilderness location, I'd suggest that you bring the person inside quickly. Can you put them in a car? Keep the body core warm.
I am not a fan of rewarming in the field. Evacuate to medical care is the best choice of action, if that is available to you.
First aid classes and CERT training will help prepare you for dealing with this and other potentially life-threatening emergencies.
Thanks for the timely question. Winter is just one season away.