One likely cause of sudden diarrhea is intestinal flu. It isn't just a smell issue, it is possibly contagious. Staff have a duty to public health not to board a guy with diarrhea down his pants. They don't have to be unkind about it, but he might not be well enough to fly. Or if he can fly, they could help him out by asking an airport store to deliver clean pants up to the gate, or call a friend of his for clean clothes and book him a later flight.