My vet book is very old, so the treatment probably not upto par with current recommendations, but one thing it mentions is anti-histamines (along with a completely dark stall). If she has true photosensitization, she'll need to see a vet as it often has anaphylactic shock reactions with it.
When I was in college a friend had a mare get a severe case of it, and she had to be put down (more due to incompetent vet, vet on call was a cattle vet and had no clue about horses). But this mare and her gelding, the photosensitization was caused by incorrectly put up red clover hay. Other plants are also known to cause it, but I don't know if horses would eat them, my vet book also lists buckwheat, not sure what else might.