M.P.
Stop the humectant shampoo and conditioner. Use a shampoo for oily hair and no conditioner. Also try washing it less. And when she does wash it, use cool water and gentle scrubbing. The less one handles their scalp the less oil it will eventually produce.
I had very oily hair and skin as a teen. Eventually, I found that washing my hair twice a week, sudsing it only once, worked best. It did take a few weeks for my scalp to adjust.