Ummmm, based on my ownership at around 20,000 miles:
Check clutch for wear and slipping (its a £800 dealer job otherwise (or about half from St Eptoe ;

)
Check rear brake caliper bleed valve for corrosion
Check battery - it should crank over 'keenly'
Check state of exposed wiring for corrosion
Check rear shock for any sign of leaks (esp. if he's a larger chap who rides two up)
Ask about alternator belt replacement (which is due around then I think)
Ask him to start it from cold - it will be noisy and rattly, but should calm down once its warmed up.
If the under seat area is clean and tidy that might show that he takes the time and care to properly clean - but no big deal really.
Mike