First and foremost, see Greepawn34's thread on castling to deliver mate!
A rook in a corner only controls squares on the edge of the board. Castling is an efficient way of developing a rook, as it moves it closer to the center.
A king on e8 can be attacked from 180 degrees. A king on h8 can only be attacked from 90 degrees. Castling gets you there quicker.
Generally speaking, a castled position is far harder to attack than an uncastled position. Good players salivate when their opponent's king tarries in the center too long.
And to be blunt, hundreds of years of chess theory have evolved to show the value of castling, every world champion has done it with extreme regularity, and all the strong players do almost as a routine.
For a new player to think he knows better than these is arrogant stupidity -and I'm sure your friend is neither, but he should be warned about appearances. I speak from personal experience, as I was in his shoes once!
If he's winning every game against really strong players, and hasn't lost to anyone, then he can get away with wearing ignorance as a badge of bravery for a bit, but good competition will cure him quickly.