Most everything you need to know is here: http://www.tsa.gov/traveler-informat...and-ammunition
When they say "TSA-approved locks" they are not referring to locks that can be opened by TSA. They mean locks that cannot be opened by anyone other than the passenger.
The passenger should print this page and bring it with him. He should also print the relevant page from his airline's website. This is the quickest way to resolve questions and hesitations from sometimes ignorant counter staff at the ticket line.