That is a very bad case of scaly leg mite. Poor girl!
Stand her in a warm Epsom salt bath for 15-20 mins to clean and soften them, then dry them and coat liberally in Vaseline. The idea is that the Vaseline suffocates the mites so you need to slather it on thick. Another option is to dip them in diesel. You will need to repeat every few days until you start to see the old scales come off and healthy ones start to replace them.
To get her to settle whilst they are soaking, make the bath in a trug or tote and just put enough water to cover up her legs... 3-4 inches. Make sure the water is not too hot.... just comfortable to hold your own hand in. Put the chicken in and drape a heavy towel over it to make it dark. That should help her to relax and not struggle to get out, but don't leave her unattended in case she falls over and drowns.
I hope you manage to get on top of it as that will be very uncomfortable for her and leaves her vulnerable to getting an infection.