A.S.
Have you tried T-Gel or Selsun Blue? THose are two of the better ones for otc. You may want to help her since you won't want it to get in her eyes. If you live in a cold environment that may have something to do with her extra dry scalp.
If this doesn't help you may want to consult a derm. She may psorsis or something in that family. It appears like dandruff but, really isn't and a dandruff shampoo may not be enough.
Best of luck.
A.