Not to sound crass, yet what will happen is that your brother-in-law will watch from the pier as you all wave good-bye to him from onboard the ship. Insurance will not cover this situation, either.
Let him know that the person checking him in at the pier will NOT be the same person to whom he spoke on the phone. The rules are very strict for boarding a cruise ship, and are governed by the State Department, not the cruise lines. (I know you know this -- this message is for him.)