Acne problems?

My little sister has really bad acne, and is really self - conscious about it. She has tried Proactiv, Neutrogena, Black Opal, and some other stuff. I even took her to the doctor, and her treatment didn't work. Her acne isn't going away. Could it be something that she's eating, like candy, soda, or fried foods? I eat those things, but I don't have acne. What should we do?