soap. Dry totally. Make sure socks are changed every day. Also if you can get shoes that are not man made materials that is great. The synthetic fibers make you sweat more. If he is always wearing sneakers try to get the kind that have mesh on the side so air can get the feet and help keep them dry, also try to buy pair so they can alternate every other day so the shoes dry out. Also if she doesn’t want to wear the powder on her feet then she can squirt some into the shoe directly. To kill the stink when she takes them off. You really will not know if something is working if you are using shoes that are so stinky already. You could try washing them in the washing machine and letting them dry for a few days in the sun to make sure you start with a pair of fresh smelling shoes. I have washed leather sneakers and they survived the washer just fine.
If all of that doesn’t work look into what she/he is eating. goes in has to come out. If it is a lot of processed foods, food high in preservative, or even foods that are heavy in spices like Indian or style foods try to work in more natural foods with less spices.