It looks like frostbite has affected the toes. Usually just above the dead area, there is swollen but still viable tissue. I would simply apply some betadine (iodine) or Vetericyn, and wait for the toes to drop off, which normally happens. If you bring her inside where it is warm, you may instead give a daily Epsom salts bath (or betadine and water) and then apply some plain neosporin or coconut oil to the feet when dry.