Tell him to get a thermometer. It will take a lot of guess work out and be easier and better for the fish. It will also help next season as temps begin to drop he can give them foods with extra fats and natural proteins (nightcrawlers ad chopped shrimp are good) to build reserves going into winter.
Below 60 deg. a cold water or wheat germ based food should be used, most stores or nurseries will carry it. Every otherday is good. Below 55 deg. once or twice a week, belw 50 stop feeding. There should be plenty of algae and micro fauna for them to eat if they want, but appetite an activity will slow down. Tell him to t