I know plenty of people who responsibly take elk on a regular basis with the .243 Win. It is a capable caliber although not an Ideal one for elk. I wouldn't use it for a first choice, but then a lot of people frown on my rifle of choice for elk the .270 Win.
535 yards is a long shot but I would't say an unethical one. If conditions are right and with a steady rest a lot of hunters who are familiar with their rifles can make that shot. I can't say much about the distance as I've taken shots on animals (mainly varmints) at longer ranges.
Combine the range and the caliber I feel that the person guiding this hunt or mentoring it wasn't doing their job properly. If this hunt even took place at that range with that caliber. Would we be so upset if the kid had used a .300 Win Mag at that range?