How's the ventilation in their coop? Often, the buildup of moisture in the coop can contribute to development of frostbite. Anecdotally, people recommend putting Bag Balm or petroleum jelly on the combs to prevent or treat it. I have several roosters with very large combs and they all seem to...
There are some good examples here: https://www.backyardchickens.com/t/1148276/frostbite. Some symptoms are black tips and/or swelling. Do you have a chicken with suspected frostbite?