Do you mean the Serial Number is not showing in the 'Device' tab? It normally doesn't show that in the device selection window, only when you press the device tab:
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Garmin will disable map updates for units reported stolen. Did he buy it new or 2nd hand? Did he register it with Garmin? Has he updated maps before without problem? Maybe he should just contact Garmin support about this if he has good title to the unit.