Ear defenders (External or internal) cut down the volume at the ear, while earpieces as used by music players and the like mask external sounds such as a shout for help or of warning. There's no comparison, and if noise cancelling earpieces are used, that's even worse.
Only insofar as it should ensure good working practices later on.
I would if he needed to be able to hear warnings or come to the aid of someone who's in need of it when they shout. These are the reasons it's against the law to wear anything of that sort on or in both ears while driving or riding a pushbike, but ear defenders are legal. Ear defenders let you hear the fire engine sirens, but headphones won't.