I'll defer to someone who may actually do a lot of music editing, but I can't imagine that you'd need even 8GB. If it were video editing, or even photo editing, sure, but unless he's working with really long lossless quality music cuts, like over an hour, I don't see it being all that useful. You'd probably get more bang for the buck getting a better CPU or even a professional grade sound card compared to an extra 8GB of RAM.
For everyday stuff or hobbyist work, 8 gigs will be fine.
Only consider 16 gigs if the person will be doing a lot of professional lossless work (which can be done with 8 gigs, although may be a little cumbersome depending on the duration and quality of audio files).