Find a clarifying shampoo. I know Pantene makes a good one. Have her wash her hair with that once or twice a week and then let her pick out something that she likes the smell of to wash with the rest of the week. Use conditioner only on the middle and the ends of her hair.
This has worked really well with dd12 and the greasies!!
My DD is going through this too. Part of her problem is that her hair is so thick and curly that it's hard to get all the shampoo/conditioner all the way out! So be sure you work with them to understand when they're "done" washing their hair.
I've settled in on a couple of shampoos that work for dd. One is pantene which was mentioned above. The other is the ausie miracle stuff in the purple bottle. DD really likes that stuff.
We've also tried the suave humectra line, since her hair's so curly. She doesn't like the smell, so we went back to the aussie stuff.