Does he want to "fix" this? Or does he want the fret buzz? For lots of the best dirty bass tones out there, fret buzz is part of where the growl comes from. For example, I've been adjusting the action on my basses and guitars for over 20 years, I know how to do these sorts of things... and if one of my basses (other than the '78 P with flatwounds on it) didn't have a small but present amount of fret buzz, I would for sure lower the action to get it. It's all about a balance between getting a good amount of fret buzz present in the sound, but not losing too much sustain, if you're talking about a good rock bass sound.
But the short answer would be "if you're looking to decrease the amount of fret buzz an instrument has, a few ways to do it are 1) use heavier strings or tune to a higher range, 2) correct any issues with ill-adjusted truss rod, 3) raise the action on the bridge or the nut, whichever is appropriate or 4) either a fret dress or a refret, as appropriate."
Probably there are other things I've forgotten as well. But the first step for you is to talk to him and see if his sound is like that because he wants it that way. Like, think of all those Korn-wannabe bands, for guys like that the choked out fret buzz is the main thing the bass contributes to the band's sound. I'm personally not a fan of that much of it, but who knows, maybe your guy is. Or maybe he just didn't even notice how his instrument sounds, or he did notice but cant figure out what to do about it. Have a quick chat with him and get to know if it's intentional or not.