This is more than just "rudeness" or "cluelessness." Falsifying time sheets is fraud. A secretary in my workplace was fired for doing exactly what this intern did--changing an already-approved timesheet. The guy is a thief, and the company is letting him get away with it, so why shouldn't he keep on doing it?

He needs to be kicked out of the intern program, and whoever supplies the interns needs to be told exactly why.