It's a judgement call, we're building a second coop at the moment and one of its features is a hospital cage for non-contagious injuries. It may be a trauma or it could be that this bird is growing very fast and needs extra vitamins, like a poultry vitamin or a baby vitamin (no added iron) in the drinking water. Certainly wouldn't hurt the others at this stage, either. Even when you're feeding correctly it's possible to get a bird with a higher demand. These vitamins go largely to the medullay complex of the pelvis and legs, in an effort to build the calcium reserves for future egglaying and adult strength.
You'll have to decide if you need to segregate. If the bird is getting re-injured, you could put a dog crate in with the flock and keep that individual in there with individual feed/water. Give her a scrap 4x4 for a low roost. How high is your roost? Any possibility it needs to be lower. Bigger the bird, lower the roost, that kind of thing.
It's true that some contagious diseases involve the legs, so watch closely. If you suspect a disease, of course get her out of there and conduct a clean-down.
If it is a nutritional demand, you should see slight improvement in 48 hours and a lot within 5 days.
