How can diet affect someone with adhd?

Rarely. See my health hints on adhd. Some adhd patients, particularly younger ones, are made worse by petroleum dyes, chocolate, and caffeine. However, the cause of adhd is neurobiological and not dietary. See http://www.Healthyplace.Com/adhd/nimh/adhd-in-children-and-adults-homepage/menu-id-888/.
Many ways. Diet can play a huge role with adhd symptoms. Sugar really does affect behavior and focus. Food allergies or sensitivities can as well. It is important to find which foods are having an effect and make diet modifications. Often, the adhd symptoms resolve without drugs.