The advice would depend on the style of music he prefers best. I can't say much about the technology behind it, but I know a bit about the sound difference.
One simple example is if he likes to play heavy metal vs. alternative/grunge style. The former would benefit from a standard distortion sound, while the latter would benefit from an overdrive sound.
A distortion sound would give you that harder, punchier distortion commonly heard in music by such bands as Metallica.
An overdrive sound gives you a wider, crunchier sound typically heard on, say, the Smashing Pumpkins during the
Siamese Dream years. Overdrive was prominently featured in the music of Nirvana, Soundgarden, Alice in Chains, and other grunge acts.
Listen to the difference:
Of course, you're likely hearing a number of other things processing that music, but the gist of it is there.
And I'm certain the likes of Martian and other actual guitarists out there will be more helpful.
I welcome any corrections or better examples.