What about shifting the thinking from the local library to a cloud service. Particularly in this case, because you mentioned that it's "backed up in iTunes", I'd look at Apple Music implemented through Sonos.

The way I read it, it would touch all the bases that you need and be less of an administrative challenge. In other words, he would just need to manage the library in iTunes, and let Apple Music do the rest. Don't have to worry about Netsync, or really even the NAS at all if you don't want.

While much less, I do have about 7k songs in my library, and I use Apple Music and Sonos and it's been a breeze. Everything just magically stays in sync, at least that I can tell so far (about 2 months in). Phones, iPads, cloud, and iTunes on PC.