Also - shouldn't speculate but if they were 3 POB in a typical four seat trainer (e.g. PA28) then some manoeuvres perfectly safe 2 POB are prohibited (Utility Category vs Normal Category due to position of C of G). Not really fair to expect a student to know that (depending on what ground exams they've taken); the OP may quite likely have seen the manoeuvres completed safely on a previous flight and wouldn't have known they weren't safe with a passenger in the rear.