One day when I was feeling pretty poor, I bought a bag of Old Roy from WalMart. My dear friend's coat within a week was nothing but a mess of hot spots. Took her to the vet. He laughed at me and asked me how much money I saved if the vet bill was factored in? Well . . .
After many free samples of premium dog food, I decided that it wasn't worth it. So, I researched dog nutrition and devised a diet of people food for her. That was a few years ago. She hasn't had a hot spot since I switched her to human-grade food. Be aware that food for human consumption can't have molds, rot, etc. in it, but dog food can. There is no regulatory agencies to monitor the quality of dog foods. Road kill is used. Rotted and moldy grains, cattle hooves, etc. Most dog foods, even premium brands, are laced with hormones that are fed to livestock to "beef" them up, sick and dying animals are sold to dog food factories . . .
This isn't just some horror story I read somewhere. It's from a friend of mine who actually worked in a pet food plant during college, and from another friend who lives near a plant in Michigan. Although many reports just echo the same thing.
Anyway, once a week I mix up a batch of dog food. Sometimes I use ground beef, sometimes chicken. Some liver, either rice or oatmeal, and a bunch of mixed vegetables. And she gets a doggie vitamin. My dog loves it. It actually costs about the same as premium dog foods. I'm convinced that human-grade food is the best for my dog's health. And no more allergies or hot spots.
Try it, at least long enough to clear up the hot spots.
Good luck to you and your dear friend.