foxmoth, for what it is worth I agree with you. If a job is worth doing it is worth doing right and to argue that it does not cause a safety issue is totally irrelevant.
If I was the student I would expect to be taught properly.
If I was the instructor I would hope a fellow instructor would point out such an error to prevent me from making it in the future and therefore improving the service I offered.
I would certainly not wish to continue making the same mistake, irrespective of how trivial.
This student may not be aware of his instructors error, however that can't be guaranteed for all.