It all depends on how much money he has to spend.
You really dont need the "latest and greatest" for audio computers. One of my audio prof's uses a 500mhz for playing back 24 tracks and it does it seamlessly, BUT if you start using a SINGLE plugin to ONE track, the pc reboots.
He could consider getting a Mac (!). Macs handle audio and video much better than PC's. I use a PC only because i dont like the feel or OS of macs. Macs are very stable with audio and most people swear by them. If you go with a mac, i can't help you much further.
If PC:
I would suggest at least 1ghz processor, at least 256mb ram, basic video card (if not onboard, dont buy anything special). I would get around 150-200gb hd and partition it for OS and Data.
MOBO: Most people I know use ASUS motherboards, but it really comes down to preference. Whatever company you prefer, go with it. With audio, there isn't really a better choice when it comes to AMD vs intel. I use an AMD, and a friend uses an Intel. They work great. It also depends on the program your using
Now, for interfacing. How does he record? Does he already have interfaces/pci card for input and output on the current computer?
Try to give me as much detail as possible with what he is currently using (equipment, software, etc).
Also, If you would like, try posting in the Irishacts.com forum under General Music. I'm part of that forum as well under yamaha135. Post a question there asking what would be the ideal audio pc/mac setup for your dad. There are many people there that can help you far more than i can.