your girl looks like she has scaley leg mites! but it doesn't look bad enough to have lost pieces of her toes yet, I know rodents can bite chicken toes at night when they are on the roost. Do any other of your chickens have injuries on there feet or legs?
if its scaley leg mites you need to smear oil or vasaline on there legs 2 times a week for 2 months! the leg mites only live under the leg scales and the oil will smother them! ALL of your birds need to be treated.