Good luck. Microsoft has put some pretty good measures in place to prevent people from downgrading (or installing other OSs period) on Windows 10 computers. You shouldn't have too many problems if you upgraded to Windows 10, but if your computer was bought with 10 pre-installed, your gonna have a hell of a time getting anything installed. I know because I tried to do the same with an Acer laptop I bought. Didn't end well. Definitely use 7 instead of 8, its a far more stable and usable OS.
Unless he "cloned" his hard drive before.
Thus, creating a VHD backup on another drive, that he can then replace the drives and load back up his old OS.
I have been wanting to do this to try windows 10 too, but not sure if I should.