Under what conditions should one diver refuse to allow another diver to thumb a dive?
Never that I can think of. After telling students in class over and over that a thumbs up means surface and then refusing to do that? I would hope that instructor would have their card yanked. It is never the right thing to keep someone under that does not want to be there. You surface, address the issue calmly, and if need be cancel the dives and go back to the pool. But to refuse a students request to surface? That's just ignorant as well as unprofessional.
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