I'm sorry, but there is nothing you can do to prevent them from falling off if the tissue is dead. I understand that is sounds terrible, but for the time being try to focus on making her comfortable and taking care that infection doesn't set in.
If you have vet care available, that would be best.
@Blooie ???
It looks like frostbite to me. Bring her in to a slightly warmer location, you don't want her to overheat and you want the feet to warm very slowly. Don't put them in warm water to warm them up.
Place her on clean dry bedding. I would suggest leaving the feet alone for a...