If he's hitting a wall and feeling the "crash and burn" feeling, that means the glycogen in his legs is depleted. His muscles will be so insanely insulin sensitive right then and there that if he eats any high GI carbs, the muscles get priority and they will suck the carbs up and store them as glycogen before any carbs get a chance to be stored as fat (so long as you don't overdo it).

As for avoiding that crash and burn in the first place with some pre workout carbs, I'm no expert but people that carb cycle have no problems with this and in theory it seems like if you take in X number of calories in carbs and then burn off 2X calories with an insane cardio session... I don't see where the big problem is so long as you don't have any conditions. Most everyone is "insulin sensitive" to some degree... but is more than 1/2 the population really SO insulin sensitive that this would have a dramatic effect on them??? I find that kind of hard to believe.