I can't see any ethical issue with the students' actions, so long as they were honestly under the impression that the professor would be creating a bespoke exam

I can't see any ethical issue with the students' actions, so long as they were honestly under the impression that the professor would be creating a bespoke exam, and that canned exam came from a legitimate source. The appropriate response, in that case, would still be to invalidate the exam due to the unfair advantaged gained by the students who studied from the canned one, but I can't see punishing those students much less disparaging their moral character. Practice examinations are routinely used to study for standardized tests, and the organization responsible for creating and administering the LSAT even sells previously administered tests directly. If that isn't cheating, I can't see how this instance can possibly be construed as such.