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The students call him professor. He is doing the job of professor. If
someone at the grocery store asks him what he does, he will be a better
communicator if he describes himself as a prof at so-and-so University rather
than as an adjunct visiting instructor at so-and-so. There are places where the
title is important and others where "professor" is truly appropriate even if
not officially sanctioned. i would agree that he should not present himself as
a professor at your insitution when searching for jobs, but his
self-description as professor in other domains seems perfectly reasonable.