"You're probably going to shit funny for a week. You are changing your diet for the first time, your body is going to adjust accordingly."
Makes perfect sense, but he had never even considered it. I read diet and workout advice all the time that's catered for beginners, and not a one mentions "By the way, you are going to start shitting funny until your body gets use to it."
Common knowledge is only shared through the community that is invested in it. With new employees, new jobs, or new relationships, missing some basic information can have terrible effects.
Also, be careful of colloquialism. I met a Korean gentleman today who thought people were literally defecating bananas in America because his boss told him "My shit is bananas".