Not much for me to add except check over any other ducks that were nearby, just in case they have injuries too. And watch for whatever it was to return. 

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The vets in my area (northeast TN) are well, not very inclined to save livestock as they are not "worth it."
The cost for getting all three injured ducks healthy was just under $600. Not cheap!
Did you change the way you manage your flock after this attack? IIRC, you were limited on the number of ducks you could have, is that correct? I remember the vet cost was considered on balance worth it because your husband is allergic to so many foods, but not duck eggs which he needs for protein.
Just wondering if culling the injured birds and starting over was an option? For $600, I think you could buy lots of duck eggs, even if you had to order them, until the new ducks started laying. Or were your options more limited?
I think the OP's duck will probably survive and do well with just home care. And if the duck goes to the vet, that's OK too. We all have different reasons for managing a home flock and most of us don't do it to save money.