P.B.
It's most definitely some kind of fungus (candida, athlete's foot etc..) so get the powder to apply on her clean feet (dry them really well because fungi prosper in wetness) and tell her to absolutely NOT wearing her shoes without socks. These should be worn once and changed, then washed with a disinfectant. I would not ler her use those shoes for a while just in case some are still in there, give them time to die out. Get her new shoes when you start the treatment so she won't get them again. Good luck.