Go to Settings > General scroll down to find what OS it's on if it's higher than 4.2.1 it's definantly a 3GS just incase he is running his 3GS on iOS 4 connect to iTunes and check if there is an update available if there is an update available as 6.0.1 it's a 3Gs. Or check at the back of the phone where it says iPhone if it has the GB model at the back it's a 3GS if it doesn't it's a 3G.
Many Thanks,
Software