I would try to get in contact with a local veterinarian familiar with poultry and hear out what their suggestions might be. Physical manipulation/fixator or surgical correction may be needed going off on how progressed the joint is at this point.
If none of those options are deemed feasible the bird may be a candidate for euthanasia although I would not pass over the options mentioned above as they can have great success.
Since the duckling was born without a fully-absorbed yolk sac there is a risk of an ongoing vitamin/mineral deficiency. Many can predispose the bird to musculoskeletal deformations such as niacin, calcium, vitamin D, and phosphorus.
In addition with B Complex, I would consider a poultry vitamin product to help potentially address some of these issues.
You've received great advice for Miss Lydia and Qautie - Best of luck.