You also might need to understand, most of us, even if we do not earn a living giving such advice, like to build systems for performance, so setting a budget may not be the best avenue for that.
I disagree completely. If the goal is to build the "ultimate performance machine" then the budget must be limitless. But most of us don't have those kinds of resources so in order to BEST use our limited resources (or someone else's - in the case of giving advice), we MUST set, and know the budget if responsible spending is also an objective.
Can you build an "ultimate gaming machine" for $1000? Of course not. But with a little homework, you can build the best $1000 gaming machine around - one that can be upgraded easily when the budget allows.
That said, there are 1000s and 1000s of parts out there, from 100s and 100s of makers, ready to be combined into millions and millions of different configurations - and for the most part, into perfectly good computers - with "new and improved" versions coming out every day.
For me, it is too hard to keep up with the latest technologies to give specific purchasing advice. Your friend needs to decide if he wants AMD or Intel. Both make great CPUs and both make great platforms. I like Intels, but there is nothing wrong with AMDs. Just do NOT buy into the hype to get AMD if you are on a budge. Yes, AMDs tend to cost a little less compared to comparable Intels, but the CPU is just one component in the computer.
Once your friend decides on Intel or AMD, then find a motherboard that supports it. Check the motherboard's QVLs (qualified vendors lists) for CPUs and RAM.
Your friend needs to do narrow the options down from millions to 2 or 3. Then we can help him decide from there.
Also, If I missed any part. Please feel free to add them.
Note the PSU is one of, if not the most critical component - when it comes to doing your homework. You MUST get a good PSU from a reliable maker to ensure reliable power to the computer. I like Corsair and Antec, but the main thing is to get enough power and that the PSU is 80-Plus certified. 80-Plus certification ensures decent efficiency across a variety of realistic loads - a very good thing.
So the PSU must be a top priority, but also your last hardware purchase - last so you can determine the power requirements of everything else first.
I also like Antec cases. And I will never buy a case again that does not have removable, washable air filters.
I don't see an operating system listed. A common mistake is some users assume they can use their old Windows license on a new computer. Understand only a "boxed" full
Retail license can be transferred to a new computer (or upgraded motherboard). It is
illegal to use an OEM license that came with or was purchased for one computer on another computer. A disk “branded” with a computer maker’s brand name, or is labeled with “OEM/System Builder”, “Upgrade”, “Academic Edition”, or "For Distribution with a new PC only", is
not transferable to a new PC (or upgraded motherboard) under
any circumstances. These OEM licenses are inextricably tied to the "original equipment". So if that is the case, I recommend 64-bit Windows 7 or one of the many free Linux alternatives. Just ensure it is 64-bit since I also recommend 8Gb of RAM.