Recognize that the person with an eating disorder will likely feel ashamed or embarrassed that you have uncovered their secret. Be firm, but caring. Be prepared to state, specifically, observations that have led you to conclude that the person has an eating disorder.
Denial is difficult in the face of hard evidence.
Don't nag about eating or not eating. People with eating disorders are already extremely self-conscious about their eating habits. Be willing to listen in times of distress. One of the best ways to help someone gain control over eating is to reach out to that person as a friend instead of focusing on her eating behavior. Stress the fact that you care deeply and would like to help in whatever way you can, including finding an appropriate treatment center and arranging an appointment.