A chickens face has lots of circulation so it's probably not frostbite and if it is she will most likely recover completely.
But your coop need to be vented no matter how cold it is outside. I know it's hard to believe but it's true. As long as your coop is draft free you can have the underside of your eves completely opened for venting. Damp is far more dangerous to chickens than cold but if you are worried about the cold adding a few inches of bedding to the bottom of the coop will raise the temperature a few degrees. You can also fill some gallon jugs with hot water and put them in the coop right when the hens go up to the roosts for the night. The warm air will rise and warm them for a bit as the water cools and they will keep each other plenty warm for the rest of the night.
You should also consider buying a thermometer so you can monitor the temperature in your coop. It's probably warmer than you think. Mount it at the height of the roost and check it in the morning right around when the hens jump off the roost to leave the coop in the morning is a good time to get an accurate reading.
Frost bite isn't painful but the black areas will probably fall off but don't pick at them or remove them since they are protecting the skin underneath. You can some triple antibiotic ointment on her waddles and comb but don't clog the little vents on her beak. Or you could just oil her and your other hens combs waddles, legs and feet once a week to help protect them from from the damp. I like vit E oil but really any oil, even cooking oil, will do.
