If he puts his data on the Mac directly he could use the external drive for time machine backups. That's one of the most awesome built in features of the Mac. Turn it on and point it at a drive and it creates hourly, daily and weekly backups ad infinitum. You can go back to any point in time and restore individual files or the whole Mac. When / if it comes time to change your Mac hardware you can fully restore the new machine or drive from Time Machine.
Plus it looks cool
