Sorry, but you've got this all wrong. Firstly, what do you want the computer to do? If you don't know this, how can you know what to buy? Do you want to access the internet? How? Do you want to play games? Do you want CAD? Do you want to have office facilities? Do you want to watch TV or films? Do you want a combination? What sort of attachments do you want? Printer, scanner, 3-in-one, modem, camera, zip drive, CD, CD-RW, DVD, DVD Writer?
Have you thought about the operating system. There's quite a lot of choice, and your decision could affect the amount and type of memory you will need.
As to your mobo, even here you have options to consider. How important is the quality of sound and graphics - this will dictate whether you can accept on-board or need expansion cards. Do you want to network?
Now for the tricky ones. Intel or AMD? Are you thinking about future-proofing, or worse still, over-clocking?
Only then can you start thinking about a mobo.
What about the monitor? What size screen, and are you thinking Flat screen or CRT - the latter can be quite cheap comparatively, if you've got the space for it.
Finally, make sure you get everything agreed between you first in writing, including costs. This can be a good way of breaking a good friendship. And, while you're about it, how about working with him - next time you can do it yourself!