One suggestion I have to help prevent future sunburn, is to treat her fly mask and sheet with an SPF laundry additive. There is a product called SunGard https://sunguardsunprotection.com/
that gives regular clothing a UPF 30 protection. I use it with all my clothes because I hate tan lines. Perhaps it would help with the fly mask and sheet.
(I don't own a horse yet and am new to the horse world, so if this suggestion is completely useless/irrelevant/inapplicable, then please pardon my ignorance)