You really want an SSD in there, even if it's only a small one to use as a cache.
I also haven't heard much about HEC for power supplies. Look for a Seasonic, Corsair or Antec if I were you.
There's nothing wrong with a 7870, but for a few bucks more you can pick up either a 660Ti or a 7950 which are probably wiser investments. If your friend isn't a big gamer, then a 7750 will still do 1080p gaming just fine, but it'll save money and be quieter/cooler.
Check out the system guides on TR. The sweet spot
isn't a bad starting place, but you probably don't need the Xonar soundcard.
There's likely very little need for a full sized case, either so you could easily go for a MicroATX board inside something a little smaller such as the Silverstone TJ08-E
or Fractal Design Arc Mini
As always, the real question is "what does he/she want to do with it?"
The advice I've given, as well as the TR system guides, are 'general purpose' machines capable of everything - but it's possible to tailor a system specifically towards gaming, media, CAD, graphics work, etc....