The legs of ducks and geese are very susceptible to injury but they often recover. Sometimes they just go down from a sprain.
If the leg doesnt feel broken or dislocated, then I would put the duck in a safe place (predator proof) and keep food and water where she can reach it.
Get some niacin pills or capsules from a health food store and add one to her food or water . You will have to either crush a pill or open a capsule. Waterfowl need higher amounts of niacin in their feed, so if she has been on regular chicken feed then she isnt getting enough.
Each day, put her in water (too deep for her to stand in) and have her swim for at least a few minutes. This is good exercise for her legs without putting any weight on them.
Ive done this with several ducks that "sprained" a leg and they recovered completely. It took anywhere from 2 days to 2 weeks.