He would be charged with theft under, and if he used the plate, unauthorized use of plates (TSA).. soooo, one whole day in jail (time served for the time spent in arrest processing), for when he pleads guilty.
Report it to the non-emergency number, and be prepared to explain how you know this guy, etc, and then that way the car crew that handles it can proceed further - especially if the plate is used on a stolen car and used for more serious offences.
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Any writings in this forum are my personal view and all opinions expressed should be taken as such; there is no implied or direct opinion representative of anything but my own thoughts on various subjects.