You attempted to refute the claim that people should control themselves better in such situations. And you did so very clearly, by inferring that spreading the mood to the 75% was perfectly logical.
As a student who has not cheated, but has had to sit through countless lectures about cheating, and a few where large portions of the class did, I ask you and (other?) teachers this question: to whom are you teaching? The ones who are learning, or the ones who are cheating?
I understand that you're stuck with both, but either find motivation in the successes or consider another line of work - it's part of the job. Otherwise you're screwing the ones who did the right thing.
People have limits. I don't know enough about the situation to know if he was under the kind of load that would cause most people to act differently, but I suspect that is the case.
As a student who has not cheated, but has had to sit through countless lectures about cheating, and a few where large portions of the class did, I ask you and (other?) teachers this question: to whom are you teaching? The ones who are learning, or the ones who are cheating?
I understand that you're stuck with both, but either find motivation in the successes or consider another line of work - it's part of the job. Otherwise you're screwing the ones who did the right thing.