Several things could be going on. It could just be a simple sprain. Pekins with their weight can have a harder time with sprains.
It could be an egg laying issue and that is putting pressure on a nerve.
I would take off the splint. Unless you know what is broken or injured, it is possible it could make things worse. You will want to gently feel along the leg and see if there are any breaks. You can use the other leg for a comparison. Notice if the leg feels hot or not. Is anything on the leg swollen compared to the other leg? Lastly, feel on the left and right of the keel bone for a string like structure. If you feel only smooth bone, it doesn't have a slipped tendon.
I would also put the duck into a deep bath where it can't touch the ground. This will take pressure off the leg and allow you to see how much it can move on it's own. If she is having trouble floating, you may need to help her.