Depends on if he is using the computer for recording or just editing using the software (Sonar).
Quiet fans, no noise from the computer unless he uses it in a separate room from recording music.
Of course the fastest computer will be the best, but a quiet one that makes in the end - no noise (or very little noise).
Cooling (I only use a normal computer for what I do) may be something that needs to be looked at also.
Not a stock perhaps fan for cooling the CPU, but in all a well-ventilated computer that is quiet.
Normal graphics card probably more than fast enough, Nvidia, AMD, but fast enough, but graphics like a gaming computer really not needed for Sonar. If plugging in an audio/interface, then depending on what kind it is (probably USB 2.0 and probably USB 3.0 although not needed but compatible for like USB2 interface), or Firewall (not used nearly anymore) or Thunderbolt (not needed for audio) unless he is doing graphics or something else like that with it, then fast graphics card and a 64-bit computer.