Do you feel it's more splay leg, where it's hard for her to stand up and the legs go out to the side, or is the leg twisted, or pigeon-toed? I would 100% get the liquid vitamin B complex going and know that it's not bad for all the ducklings to get it, so you are probably fine to leave them all together as long as she's not being trampled. I had a pekin with splay leg that developed about 3 weeks old and I just treated her while keeping her in the brooder with the other girls. In her case I used vet wrap to pull her legs into proper position, and dosed her with liquid vitamin B complex for many weeks, let her swim for ten minutes or so a day to stay strong, and she eventually regained her leg strength. I'll post a video here so you can see what her hobbles looked like after she got stronger and was walking with them: