I think this an important possibility to consider. Ducks hide serious illness very well. By the time they show obvious symptoms, I have a sense of urgency, based on prior experience.My first thoughts are keep here and the others out of the pond. This is most probably where they are picking up the leeches. I think you should also be checking the other ducks as well and any other fowl who may have had contact with this duck or the water. If you can offer a kiddie pool or some such thing so they can bathe and somehow keep them from the grounds surrounding the pond. Not easy, I'm sure.
Another thing to think about...if this is also a case of peritonitis, an antibiotic may be needed. You will need to watch this bird very carefully to see how she is progressing. If the stomach doesn't go back to normal size soon, I would highly reccoment a dosing of either Sulmet or Di-Methox. Whichever you can get. The Sulmet is a bit more harsh than the Di-Methox, but then so again is the peritonitis. The reason I am suggesting either of these is to kick the infection back and also to kill and purge any remaining leech infestation. If you do need the antibiotic, please follow this up with a good poultry or human probiotic. This should mitigate any stomach upset caused by the drug.