For those not familiar with crew training.
The examiner was sitting in the right seat, he was doing a check for the transition captain sitting on the left. The guy on the left seat was doing his command training.
The examiners son had a CPL and was not type rated on the 737.
The examiner probably got up from his seat (the right one) during cruise and let his son sit there... enjoy the view, keep his seat warm whatever.
All 3 pilots (2 type rated) were in the cockpit. Even though the non type rated chap didn't actually touch anything, there are obvious safety and legal ramifications.
Company and legal policies state only authorised people are allowed in the seat, the son could be an ex navy carrier pilot- it doesn't matter.