While I wait for my headphone amps to arrive, I am currently running my Grado SR-325s from amplifier / pre-amp headphone jacks, and the results are quite acceptable. The lows, mids and highs all have good definition and presence. Then again, the pre-amp is a THX "ultra certified" component (whatever that means) and the amp in my bedroom is a British-made stereo amp, so I doubt the headphone gear on either beast is too shoddy. What is your friend using?
As others have said in other threads, a headphone amp will certainly improve the sound quality coming out of your friend's Grados. It depends on whether he also wants to hear fine nuances or is just happy to rock out. An amp that is well matched to the Grado SR-325s will pump the sound up more and exploit this heaphone's particular virtues - speed, clarity, bass - while also bringing any shrill treble under control.