my advice is: Fill a tub of water (lukewarm) and then add: 1 tablespoon of Epsoms Salts available @ supermarket or Pharmacy for about $2 a box; put the Duck in the water and let her sit in the water for atleast 5 minutes making sure the leg is immersed; no need to get her face wet;

The Magnesium in the Epsoms Salts is uptaken by the "skin of her webbed feet; and circulates; this relaxes the Duck;
this is how you treat the duck; with Magnesium and water...and rest; keep her in a cool shaded area for a day
(repeat this 4 or 5 times daily until duck fully recovered..this could be a consequence of a bug bite; give extra feed and greens; greens or pellets high in Magnesium!