Please come back and let us know that she is already under a doctor's care for seizures! That's going to be anyone's first thought here -- the seizures are the bigger issue.
Once we know she's getting professional care and monitoring and maybe medication: I do think you need to tell the doctors about the sheets. Bear in mind: The issue may not be that chemicals are leaching into her body from the sheets; the issue may be that whatever is causing the seizures is causing her to sweat excessively. Unless she has a very specific sensitivity to the dyes in the sheets, the likelier issue is not that the sheets are harming her but that she's causing the sheets to fade.
Are these brand-new sheets or very recently purchased--? If they've been around a while they should have been washed enough times that the fixatives that make new, unwashed sheets stiff are all washed out. If they are new and not washed or washed just a few times before she started using them -- get her sleeping on the oldest, most washed-out sheets possible. Ask the doctors about all this.
A thought: The dye issue may be totally unrelated to her seizure condition or to the meds she's taking for that (if any). Does she wash her face or any part of her body with any washes that contain acne medication? Especially benzoyl peroxide for acne? Or even salicylic acid. Acne creams, lotions and even just face or body washes that you think "wash off" and supposedly don't stay on the skin definitely will bleach areas on sheets and towels. Our color towels have large white areas on them from my teen drying her face after washing with a benzoyl peroxide face wash. Check all products she uses on face, body or hair.
I hope the doctors can help control her seizure condition and she is doing better!