Yes, you can file frets down (that's what the mean by crowning) but I wouldn't advise you to do it by yourself 'cos if you do it wrongly, even by just a little, it'll affect all the rest of your frets as well. Worst case scenario is that you keep doing it wrongly so you keep filing all the frets till there's no more fret left.... then you'll just have to re-fret the whole thing, which may turn out to be a very unprofitable exercise depending on the value of the bass.
I seem to get the idea that you don't understand where fret buzz comes from though... my apologies if I'm mistaken. Let me try to explain it to the best of my knowledge then.
Fret buzz can be the result of many things - these are the most common
1) The frets are not completely level (i.e. worn frets, bad fret job...)
2) There is too little relief in the neck (i.e. neck is too straight)
3) The saddle is too low
4) The neck is warped...
Anyways, so you can see that filing down a fret or two may not be the solution to your friend's dilemma. Tell him to figure out what's wrong and then work his way from there.