Agree it has to start from the inside out and won't happen overnight.
First and foremost she should be current on whTever de-worming program the vet suggests for your area and her living environment.
When one of my TWH's had huge flakes of dandruff that even the medicated shampoo couldn't keep rid of, he had environmental allergies. Along with the big flakes, his skin would crAwl and he did not like to be groomed unless it was the vacuum.
What got him under control was Remission which is high in magnesium. He was also always a horse full of angst and the Remission calmed him to where Mr. WTW thought he was a new horse
Not saying this is your horse's issues, I am just saying environmental allergies were what was wrong with my horse (the big dandruff flakes), etc.
Diet is also important. My horse only gets the Remission, Probios, flax, and a condensed vit/min supplement mixed into two cups of Timothy pellets twice daily.
His coat is slick without that awful itchy dandruff and he now enjoys being groomed