Frostbite usually occurs when a foot or toes get wet when temps are below freezing. I don't know how large your coop is or how close she was to a heat lamp, but her foot either has frostbite or a band around it that has cut off circulation. It looks more like frostbite to me. Let us know what happens. There are several threads about frostbitten feet if you go to the top of this page and enter that in the search box. Frost is one little chick in those threads that lost both feet about 2 years ago, and was living well after that.