He should have said on the WhatsApp group, right at the beginning, "I am your professor,and I monitor this gc. You can use the gc to discuss the class and ethically help each other, but if you engage in exam and quiz cheating on here I will see it, and I will fail you. If you open or join a new secret gc to circumvent this, I will find it and I will fail you. Do the work.".
If they cheated after that, immediate ban. Cut them from the course. Stop wasting everyone's time.
Instead he got off on spying on them for months, buffing his ego with how good he was at using tools to measure the cheating, had to actually change the course and the final because he was letting the level of cheating get out of hand.
He's like a boss who gives you zero verbal feedback all year, maybe only positive feedback, then sticks you with a bad annual review, listing behavior that would have been easily corrected had it been mentioned.
Yes everybody knows they are not supposed to cheat, but he let cheating become the way things worked "in practice" for his class, and then punished them after wasting a lot of their time and money.