If you are feeding exclusively, then that is way too long to go without pumping or feeding the baby directly. You might accidentially ween the baby and she may get blocked ducts or worse.
If she has a way to get some privacy and doesn't mind pumping in public, if you have a good battery operated pump with a powerful suction (not powerful enough she'll get blocked milk ducts or leak everywhere) or one that can plug into your car lighter you can plan breaks for pumping in your car if baby isn't along with you. Baby is better of course, but if she pumps every 2-3 hours for at least 10-15 minutes or at least at the same times of day her baby feeds and until her breast are fully empty (can't pump anything more out) she'll be fine.
Be sure to bring a cooler that can keep the milk stored at 40 degrees or less to keep it safe and useable if you will be leaving the milk in the car. I don't trust just using ice packs and an insulated cooler in a situation like this, since it's summer and very hot in a car.
There are small electric coolers that you can plug into your cigarette lighter that will keep things cold at about 40 degrees. You can get them at Target, Walmart, or any store that sells camping supplies.
Be sure she drinks plenty of water and doesn't get heat stressed or dehydrated. That could mess up her milk production for a few days.
For similar questions you can get answers from the La Leche League website at www.llli.org