Actually, it could be that they violate the NDL for that computer at the user selected conservatism. If that were to be a violation of the base algorithm on that computer, that’s a different discussion.
That was a good example. 48 hours does seem to be pretty severe, especially when the dive itself was actually within the NDL. That is a good lesson for him to learn. Missing two days of diving for an error is definitely a lesson. But, there was more than one mistake there. First was not setting the correct mix. Next was just following the dive guide, and not paying attention to/understanding the computer.
Not sure there is a real good solution that would be commonly accepted today. One option would be to dive tables, assuming the diver is familiar with their use. Depending on the dives, that may not be ideal and could limit the other divers unfairly.
If he had a computer that wouldn’t lock out, there could have been another option. He may have had to sit out the next dive until that computer would allow for a dive of the planned duration. That sounds better than sitting out for two days.