Apparently the customer’s hard drive had failed and a new hard drive was installed. When they picked up their Mac they were given a separate hard drive, which presumably was the old hard drive from their computer. When the customer got home, they decided to try to mount the old drive via a USB enclosure to see if any files could be salvaged and discovered that the drive contained, not his files, but Time Machine backups of internal documents dating from late 2009 through May 2011.
Cult of Mac was originally approached by the customer with an offer to purchase the hard drive; however, they did the right thing and refused, instructing the customer to return the drive to the store from whence it came. Good for them.
image (c) The Cult of Mac