Looks like bumblefoot, glorified word for a staphylococcal infection of the foot in poultry. The funky scab is probably where she was injured, and will be a good place to focus your attention should you elect to treat her yourself, at home, otherwise a veterinarian could perform the minor surgery if you have one that's willing and able to help with your chicken. Chicken pus is different than human pus. Theirs is more solid, cheesy, waxy, instead of liquidy, so you can't just push a pin in it to drain the infection (like you can sometimes do with humans). There are MANY great videos on youtube and some are linked here at BYC in previous posts about bumblefoot. Watching a video of the surgery may help you decide if you want to perform this at home or seek professional help.
Bumblefoot is totally treatable if caught in time (and it looks like you've definitely caught this in time), but can be fatal if left untreated.