Well, the first think you need to do is pen and control the duck until you can actually identify the wound. You can clean the area. Any external parasite or injury would be apparent as just that, something related to a flesh wound. However a genetic or nutritional deficiency or an internal parasite would not be as visible. Clean the wound with water and non-antibacterial soap. Then put on non-painkiller antibacterial ointment and wrap the feet using first gauze then that semi-sticky medical wrap bandage. Yes, you will have a mad duck, but it will give her a chance to heal.
Confine the ducks to a safe place for a week, rake and mow the lawn that they play in normally and check for any anomalies. It is quite possible that something is attacking her---like a rodent, too.
Ducks are considered the lowest maintenance poultry often because they just do not get sick like chickens, guinea hens and others. That said, once they reach a certain point a vet may be your only option.
