It is not standard procedure at all the instructor should have been in the cab with the trainee,
Exactly. But not always practised for various reasons.
The idea when training is to recreate the test procedure so to stand outside the truck while teaching the reverse but to sit inside the truck while practising the braking exersise as would the examiner.
However I often stayed outside the truck while teaching the braking excersise so that i could
A/ Watch how straight the truck was on stopping
B/ giving the trainee more responsibility in the cab alone.
Many trainees rely totally on the instructor to tell them when to brake and how hard etc . I used to go through it with them until satisfied that they had a good understanding of the manouvre then get out and let them do it with out me sitting there.
All instructors have different methods and as long as it works then there is no saying what is exactly right and wrong as long as the point is put across and the trainee passes then the instructor has done a good job.
So yes, technically he should have been in the cab but in practice its a matter of choice.