Do I agree that some student should have said, "Hey - I've seen this"? Yes, pretty much. But what then are the consequences? As I see them
1. 199 people who think they are in hog heaven are furious with you for ruining their good thing.
2. 400 people who have put serious time into preparation see their planning ruined. (Yes, they should be studying; but also, their time is finite, and they spent time preparing for this exam on this date, which they might have spent otherwise.)
Would I have had the nerve (moral fiber, whatever)? I hope so, but don't know.
3. 199 students + 1 are not spared the inquisition.
It seems unlikely that you as an individual would know how many other students in the class had access to that same guide. Regardless, I don't think it really matters; they should have notified the professor.
It's not unreasonable to extrapolate from "got it from a friend-of-a-friend" to "quite a lot of people will have this".
Yes, one should notify the professor; but I think it's acceptable to do this a little less publicly (e.g. by writing it on the test or sending an e-mail), if one so desires.
1. 199 people who think they are in hog heaven are furious with you for ruining their good thing.
2. 400 people who have put serious time into preparation see their planning ruined. (Yes, they should be studying; but also, their time is finite, and they spent time preparing for this exam on this date, which they might have spent otherwise.)
Would I have had the nerve (moral fiber, whatever)? I hope so, but don't know.
3. 199 students + 1 are not spared the inquisition.