I distance run/cycle on low carb, have been for about 8 months. I have found that it took me several months before my body adapted to intense exercise in ketosis for more than 2 hours and that I would crash hard without carbs. Now, I can do a 4 hour training run in ketosis with only taking in coconut oil chocolates or meat/protein shakes without any problems. When I race however I still carb load and things like gels really do give an extra boost (although the grains reek havoc on the digestive system). I do most of my training, except for my really long training runs, without carbs.

Depending on how long his bike is going to take him and what his goals are, a moderate carb plan might be the way to go.