Has she always had it? It could be something more than the products she uses. It could be psoriasis of the scalp, seborrheic dermatitis (thank you spell correct!), cradle cap, or yeast. The reason I think it's more than dandruff is because you say it's crusty. I don't think dandruff is crusty, it's usually flaky. You can have them take her to a dermatologist. But, if they don't want to do that, they could always just take her to her normal dr. He could tell her if it's more than that.
It's likely dermatitis or psoriasis and not products build up. (Though which products they use on her hair might make a difference). She'll probably have to use special shampoo or something. I recall that you can even have it on the scalp and not on the skin. Even in the eyebrows.
I hope they find out what it is! Good luck!