Try some tea tree shampoo. And give it a good scrub (especially in the back about two inches above her hairline). I find that in the summer, my oily hair makes my hair smell bad and this will help.
Also, Neutrogena makes a clarifying shampoo. Try using that once a week. Also, she can touch up her roots with dry shampoo if needed. Mu hair is blonde so on occasion, I sprinkle baby powder into my roots and let sit for 10 minutes then brush out.
She also might be over washing - too much shampoo will make her scalp over produce oils. So only one wash per shower.
Most people I know only wash their hair every other day or so. I'm so jealous. People would run screaming if I came into the office with unwashed greasy hair!