Sorry to hear about your duck.
Heavyweight breeds, especially Pekins, are quite prone to leg problems due to their heavyweight. Providing them with soft grounds, a deep water source, and a pen that is free of things they might trip on can greatly reduce these problems from occurring.
As far as treatment of the sprain, start by moving the duck into its own pen as to reduce its risk of reinjuring the leg. In addition, you can start adding some supplemental B vitamins to his diet to support good leg health and start soaking the leg in warm Epsom salt water for a few days.
If the swelling doesn’t subside soon, starting him on an NSAID might be good. Meloxicam, Carprofen, or ketoprofen can be used with ducks, but are RX only, if you do have some baby aspirin you can add one 325mg tablet to one gallon of water.
Ultimately, you can follow the treatment above, but what's going to be needed is simply rest and time to fix the problem.
Thank you very much for this info, Isaac. I have had her caged in the house since Saturday afternoon. I’ve been doing everything you mentioned except for the medicine, which I do not have. (Except for meloxicam -for me- but I don’t know a dosage.) I am going to call the vet this morning and ask for some medicine and/or dosage for the meloxicam I have, even if I don’t use it for her this time, I will have it on hand for the next time. (My BO has already had two different sprains, but not as bad as Petey’s.)
Until this morning, she has been very quiet — which I will say is very
unlike Petey. Not only is she quacking, which is normal, she is panting pretty hard. She is not hot, as it is much cooler in the house than outside. I would think if she was in pain, she would be more silent. Ducks are new to me, but I am thinking it could possibly be she might be trying to lay her third egg.

I am waiting a bit before I take her outside to her pool for some water therapy and to see her buddies, just in case an egg appears. I just don’t know enough about the duck’s egg laying schedule as it seems to be a little different than a chicken’s. I’ve got to research that.
More about her habitat here. I probably do not have the best setup for ducks. I’m realizing this in hindsight, of course. My ducks do free range with my chickens. They have total access to several acres of my yard, which a majority of it is on a slope, including their run which is in my upper back yard. The only real negatives include, first of all dang hickory nuts that fall faster than I can pick up, and unfortunately I do not get them all. These nuts can easily be stepped on causing the “stepper” to fall. Yeah, ask me how I know!

Anyway, I’ve seen the ducks and chickens step on these at times.
And secondly, my back yard slopes into a deck. There are four sets of steps that access the deck from the yard and the ducks have found them. They love to come on down to the house and hang out at my back door and on the porch of the dog house. Not thinking that ducks could maneuver steps (stupid me), since everything I’ve read said they needed a ramp, I didn’t worry about them even trying to reach the deck. Dear lord, I’ve learned that a duck can do just about anything they darn well please. They
are a different animal for sure. Talk about a strong-willed child!
Even though either of these dangers could have caused Petey’s leg injury, she was definitely walking fine and uninjured when I locked them up Friday night. It wasn’t until I let them out Saturday morning that I noticed her very significant limp...along with my first duck egg. A very sweet and sad situation all rolled into one.
Unfortunately, I realize going forward that I most likely need to either
not keep ducks or only keep the much lighter breeds, like my runners and WH. My BO and the Pekin mix ducks don’t fall into that category.
I’m sorry for the long post. I tend to type too much, but want to make sure everything is disclosed when it comes to a hurting baby — whether it’s a chicken, duck, dog, cat, horse, or a grandson!

Thank you for taking your time to read and respond!