Oh your poor chicken looks awful, Its sad people will let that happen and then even sell it in that condition. I hope you are able to get her legs back to normal.
Everyone needs to remember it doesnt matter if bought from a very trusted source like a friend or maybe a swap or an auction, no matter the source
QUARENTINE QUARENTINE QUARENTINE. No matter what for at least 30 days, longer if possible.
I would spread DE powder or even something stronger like sevin dust garden powder around the coop and yard. And then keep an eye on everyones legs. Keep your new chickens separate as far from your existing flock as you can for at least 30 days just incase they have other underlying sicknesses you haven't noticed yet.
Keep us posted on the progress on her legs.