Poor weight could have had a role in precipitating the frost bite.
I would cull her, but it is possible she can be salvaged. If she is to be salvaged, then isolate her from balance of flock and keep her on a clean and dry surface like provided by hay. Protect her from worst of cold and wind until weight is restored. Give her and extremely palatable diet. My first feed would be a moistened mixture of layer and grower feeds mixed roughly 1:1. The look into possibility of worm infection, and treat accordingly. You need to get her weight back up so she can fight off the infection were dead toe parts are necroding.
Show how she walks and what her face looks like as they can give a good handle on some other health issues.