My main triggers are bright light and unexpected loud noises. Real good sunglasses and a hat with a brim (I usually wear a baseball cap). Also, proper hydration! And like previous poster said, keep as close to meal and sleep schedule as possible. The meal schedule is usually important for me. When it is hot outside, I am usually not hungry. I have to force myself to eat something every few hours just to keep sugar levels regular, even if I'm not hungry. If I don't, I get slammed hard! Not fun at all when on vacation!
I have also found recently that a stuffy nose/head have been added to my list of triggers. Took a while to figure that one out. Even if I can breath clear, my head sometimes gets stuffy. If I don't take a decongestent as soon as I ralize my head is foggy, I end up with a migraine.
The best advice I can give is talk to your mother. Find out if she has a list of triggers, and what she usually does to try to avoid them. If bright light does, try to stay in shade, wear hat and sunglasses. If loud noises, maybe bring along a supply of earplugs. If fireworks are in the plans, have her put the earplugs in prior to them starting. They won't block out all of the noise, but will reduce it to a point that should be ok. If she's not sure of the triggers, then she should really talk to her doctor to help try to figure them out (not just for the trip, but for everyday living and comfort). Might have to chart the migraines for time of day, what she was doing and what was going on around her. Any possible food triggers ? (My DH is allergic to onions-that is the only trigger for him, but man does he suffer with even the smallest piece of onion!)
If she usually has warning signs, make sure she tells you as soon as she can so you can take a break, she can take the meds or whatever. And most of all, relax and have a great trip. Don't push too hard, too much stress can also be bad!