In this instance the seating issue shouldn't have made it to the aircraft, traveling as a family he should have had it sorted at checkin, checking in at a kiosk doesn't permit seating coordination, it's pot luck. Traveling as a family of five we leave seating till we put the bags through, staff have always managed a row plus two across the aisle or row in front/behind. Don't know if having frequent flyer cards helps, or the fact we are always early. Checking in late is obviously going to seriously curtail your options.