So for healthy, non-diabetic people it really doesn't matter.
(With the obligatory addendum that going all "low-carbs" or "keto" is a very, very bad idea if you expect to do any exercise at all)
Now, in reality, energy during exercise is always derived from a mix of both fatty acids and glycogen. In general, as exercise intensity increases (you are getting closer to VO2max), a larger ratio of energy will be derived from glycogen. So for your friend, running 54k of mountains is only going to be done at rather low intensity (50-60% of VO2max), where ~60% of energy will be from fatty acids, saving glycogen and preventing hypoglycemia.
Hypoglycemia, incidentally, is also known as "bonking". It's rather unpleasant. On long, high intensity exercise sessions you can't usually replace all the glycogen you deplete through food intake. So if you start with lower glycogen stores through a low carb diet, what you are doing is priming your body for hypoglycemia.
http://eatingacademy.com/how-a-low-carb-diet-affected-my-ath...
Sugars really only have several places to go once they are ingested: Held in the fiber of undigested food, blood stream, liver and muscles.
We do really well with vegetables even though they are carbs since veggies are mostly fiber and very low carb. The high glycemic foods are okay provided that muscle glycogen is going-to-be/has-been depleted in the form of physical exertion.
Building muscle would also increases ones capacity for storing said glucose, but it's not easy or popular since the rate of muscle growth at best is about 1-2lbs/month under ideal conditions of physical exertion, nutrition and rest.
However, exercise and low-glycemic diets do explain the efficacy of reveesing t2 diabetes.