That's the first thing to suspect, along with worms. Your friend can safely treat this fellow for both worms and coccidiosis at the same time. Mix up the coccidiostat (Corid if that's the brand sold there) and put a teaspoon of sugar in the water bowl so he will be sure to drink it. It will also elevate his glucose and make him feel instantly better.
His crop should be checked in the morning before he eats or drinks to rule out a sour crop or impacted crop. If it's full after a night of not eating, he has a crop issue that needs treating.