Could you post a few pictures of the area? If you get a video of him trying to walk, that might be helpful too.
There are a couple of conditions that could cause a swollen joint - as Azygous suggested MS is a possibility. Although the skin may appear intact, a simple fine poke into the skin can allow bacteria to penetrate inside the tissue and cause an infection. Recently, a member had a similar problem and through fine need aspiration of the swelling done by therevet, they were able to determine the swelling was due to a Pseudomonas infection.
As far now, I would try to relieve weight off of the foot. That can be achieved by letting the duck bathe in deep cool water supervised for a while each day, or by making a sling. When it's not in either one of those, the grounds should be soft to reduce the chance of pressure sores.
B vitamins are often helpful in leg problems; you may consider adding nutritional yeast to her feed or give her a 1/4 of a B complex tablet, or a few drops of Cattle B injectable Complex every few days.
If you have a vet near that's familiar with poultry, it may be best to take here there so tests can be done to determine if there is a break, or whether there is some sort of bacterial infection. C&S can be done to determine which antibiotic would be helpful in the case of a bacterial problem. Otherwise, you can treat by an elimination basis and try an antibiotic yourself such as enrofloxacin, clindamycin, etc. All can be found online in fish, or bird form.
