Migraines can really put damper on being in the parks. Mine have mostly been under control over the last several years, but Illuminations has caused me to have them twice. The best advise I could give would be to stay hydrate, monitor precursors, and make sure to have medication on hand.
If she was to get a headache, would first aid be able to do anything to help? Like, maybe offer a quiet, dark place for her to rest until her rescue med kicks in and I can get her back to the hotel? I'm especially concerned about what we would do if she is to get a migraine while at the park. It is just she and myself traveling--husband and baby sister are sitting this one out at home--so I won't have anyone there to help me.