I don't know if this will help in your particular situation or how much you would give to a duck of this size, but I have used .25cc of Gentamycin administered as a shot to a Call drake for 7 days straight for prolapse, and it did take the ugliness and fluid swelling down. He's better now and "keeping his pride tucked" where it belongs. Gentamycin is usually used in mares that are bred to remove fluid out of the uterus, and it's an antibiotic. So I don't know if that would help or hurt your situation here, I've never had to deal with this serious of a frostbite in anything; I think it could help but you should run it by a vet for the final okay if you decide to do this...they would have to sell you the Gentamycin anyway and tell you if you would need to increase the dosage for this duck, and they might have to help you if you don't know how to give a shot to a bird. So talk to the vet and see what they think.