Does she have fine hair/coarse hair/curly hair/...
When my kids needed to start washing their hair more, we switched to a deep clean shampoo. Now their hair can tolerate it because they have pretty fine hair (if her hair is coarse, not best option).
Deep clean can also be called volumizing. It just means it doesn't contain anything that will weigh the hair shaft down (like conditioners).
Avoid 2 in 1's - they are not good with oilier hair types.
This is assuming she has oily hair.
If it's simply a matter of not washing her hair correctly, you really only need to wash the SCALP. You rub the shampoo between your hands and add a teensy bit of water to dilute it enough to make it spreadable, and then rub that into your scalp. Lather up, then rinse.
The shampoo going through the rest of her hair will be enough to clean it and not dry the ends out further.
If you scrub shampoo through the length of your hair, you will need conditioner on the ends. Better just to let the soap wash through the ends on the way down as it is rinsed.
Conditioner - only on ends, or below ear level/chin level if really knotted.
Hope that helps. I would only try the volumizing or deep clean shampoo after you make sure she's washing her hair correctly. Getting it properly rinsed is important too. Mine sometimes forget that part and it will look lank.
Good luck :)