How are her finger nails? If they are breaking and slowing growing as well, she may need more Biotin. Sources of biotin include high protein foods, such as liver, salmon, sardines, pork, avocado, cauliflower, mushrooms, raspberries, bananas, cheddar cheese, nuts such as peanuts, walnuts, almonds and pecans, soybeans and other legumes and whole-wheat bread. Food-processing methods destroy biotin -- unprocessed foods are a richer source of this vitamin.
Sometimes it is just genetics. The suggestion of not washing too much is a very good suggestion. Once a week is enough as long as it is manageable. That way her natural oils can build up in her hair and maybe it will not break so easy.