Harris gave some great suggestions. I would also suggest (if he is not doing so already) consuming some calories during the run. That 15-18 mile wall is very common in marathoners and is often caused because your body runs low on carbohydrates (your body’s preferred energy source during exercise) to break down and use for fuel. I recommend consuming something like a GU Energy Gel during the runs. They are designed to replenish the nutrients your body runs low on, including carbohydrates, during long bouts of exercise like marathons. He should practice using them in workouts before trying it in a race.
I hope this helps,