Although perhaps it could have been phrased a bit more "polite", Scott is correct. I remember back in the day trying to do audio with a coupld of Dell generic boxes. I had nothing but trouble, a key program crashing, etc. Some parts simply don't play nice with others, and the needs for audio are very different than the needs for gaming. That isn't to say it can't be done, but it is less straight forward than one might think.
As far as what your friend needs, more information would be helpful. What he wishes to accomplish will largely dictate what would work best for him. Particularly if he uses lots of VST's with heavy disk streaming, coupled with lots of effects, or something like MIR Pro, than as powerful a machine as you can put together would be wise. For heavy VSL use somethin like a 4930k, 32 gigs of ram, and most samples placed on SSD's would be helpful.
What sort of soundcard will he be using? How many parts does he wish to record at once? Is he recording just himself one part at a time, or is he recording an entire group? Or, is he doing everything ITB? If ITB, or only recording himself, a soundcard with fewer inputs will be enough, while if he is recording an entire band, a more sophisticated card will be helpful. Also solid drivers, and the ability to track at low latency will help. As far as soundcards go, RME is as good as it gets, unless one spends much more, and their drivers are a known good thing. Having a great computer coupled with a poor consumer grade type of soundcard rather defeats the purpose.
What sort of speakers is he mixing on? Is the room he is working in treated or untreated? What are the dimensions of the room?
It might be that spending a bit less on the computer, and budgeting a bit of money for other areas might be helpful.