I agree it looks like Frostbite.
Even though this is a chicken, "treatment" would be the same for Ducks. The hen did loose both feet, but she's learned to manage somewhat. The OP outlined what she did in the first post. Soaking every other day in Epsom salts and keeping feet clean/dry/wrapped with antiseptic spray (I assume something like Vetericyn). The Epsom Salts may be "drying" but I would make the assumption that's what you want when tissue is dead like that, it's going to dry and fall off anyway, so it may help promote that. I'm not a vet nor expert, so doing some reading of severe frostbite cases on the ER forums and seeing what others have successfully done would be your best bet.
https://www.backyardchickens.com/threads/frostbite-lost-both-feet-my-hens-story.1343701/
Even though this is a chicken, "treatment" would be the same for Ducks. The hen did loose both feet, but she's learned to manage somewhat. The OP outlined what she did in the first post. Soaking every other day in Epsom salts and keeping feet clean/dry/wrapped with antiseptic spray (I assume something like Vetericyn). The Epsom Salts may be "drying" but I would make the assumption that's what you want when tissue is dead like that, it's going to dry and fall off anyway, so it may help promote that. I'm not a vet nor expert, so doing some reading of severe frostbite cases on the ER forums and seeing what others have successfully done would be your best bet.
https://www.backyardchickens.com/threads/frostbite-lost-both-feet-my-hens-story.1343701/