As a musician using two notebooks(Core 2 Duo), you'll want to stick with Windows 7 as it is rock solid and Avid sucks for Pro Tools certification of new OS releases--some audio interface companies usually have issues with new OSes too. If he uses Cubase & Ableton they're likely going to patch existing release to support Win8--Ableton v8.4+AmpliTube is working on my Thinkpad w/Win8. Personally I use a Mac+Logic for heavy recording duty as Avid hardware certification of desktop components is a pain on Windows and why I still use a old trusted "certified" Thinkpad T61 for Pro Tools.
With that said a big SSD(256GB) for boot drive and apps is ideal(you could scale back to a low-end i5), two WD Black HDD for recording storage and one WD Black as a "scratch drive". Use a PCIe 1x slot for extra room to use more SATA/eSATA HDDs.
--Motherboard: make sure it meets the requirements/certification for Avid Pro Tools or you'll learn what a bag of hurt PT can be
--Most musicians prefer having at least one onboard firewire port and also a PCI card for extra FW400/FW800.
--PCI/PCIe or Firewire Audio Interface: this is much better latency wise than USB 2.0 interfaces and onboard sound input/output is garbage even for headphone usage.
--Video card... hmm, I'd lean towards nVidia consumer cards as their drivers are solid 99% of the time like their Quadro but if he wants/plans to use three monitors an AMD FirePro V4800/V4900 is the way to go as drivers are much better than consumer cards.
--Processor: If Pro Tools is being used with midi virtual instruments more RAM is a better investment than raw CPU power.
---Additional notes: Don't build an entertainment PC as it brings more woes into a working environment, you want something with long term driver support like business PC makers(Lenovo ThinkCenter/Thinkpad or Dell Optiplex/Latitude). Less bling/eye candy makes a solid recording studio setup.
If video card demands aren't big on his list, Intel IGP is good enough for a two monitor setup... a Mac mini w/i5 would be a good "benchmark" spec to start from.
There are two types of musicians in my opinion:
1) A single instrument musician who DJs on the side for rent money and changes bands as many times as his/her underwear in a year. Specs of a computer isn't going to be as demanding unless they heavily use virtual instruments/loops+effects, I know many DJs who still use Core Duo notebooks with high resolution screens.
2) A multi-instrument musician who attempts to record every individual instrument track him/herself with minimal virtual instrument(midi keyboard or loops). Often won't use software effects, a mid-range desktop/notebook can last 4-5+ years and sticks to using better gear to get the job done(ex: guitar/bass pedals, custom microphone mounting setup for drum recording, etc).