Oil Pressure Relief Valve

If the motor feels smooth to turn over, you could do a 15-minute ground run and then drain an oil sample and send to Blackstone Lab (or similar) for analysis.  Bearing damage would likely show up as high wear metals.  This may not be enough to make a full determination, but it would be a good indicator. If the report is bad, then at least this confirms what has to be done. 

Before any further operation of the engine, you also want to make sure all the lifters are pumped up. If it was actually run with no oil pressure, some of the lifters may be compressed.  And if no lifters are compressed (flat), that's an indication the engine was still getting some oil. However, if you have already run the engine with oil pressure reestablished, the lifters may have already recovered.  

I am curious if your lad remembers any strange noise from the engine while running it with no oil pressure. If any of the valve related components failed during this period I believe they would have shouted at him loudly. 
I am with Jeff, if a manual turn feels real smooth make sure oil is everywhere it needs to be and give it a try. You then could extend ground op with all sorts of power setting to a point step by step.
At some point if you developed a positive perception on the physical health (monitor all vital parameters closely like revs, temps, starting behavior, sound, etc.) of the engine you need to make a decision.
In case the engine is dead already you can hardly kill it a second time. So while being sad about the incident itself one can consider the worst case as an invitation to try some things out carefully before taking further steps.

I do hope your engine only aged a bit more/faster than normal during the 5 minutes. The 912 can be real tough!

Jeff B I like this idea.  I am familiar with the tappet test with a rubber mallet and feeler gauge.  Are you saying that the tappets will stay pressurized after running the engine then sitting for a while?  I couldn't find anything in the maintenance manual that states that, just the purge procedure.  Are you suggesting to do the tappet test before purging or after?

Yes they will stay pumped up while sitting - generally.  You know from listening to car engines that every now and then you get a lazy one that drains and then takes a few seconds to pump back up once the engine is started.