yeah, I've done repairs in the past when boards have gone bad and just swapped them out for identical models. Since you and your friends systems are probably not 100% identical, you might run into some issues with stuff like graphics card drivers, though if you completely uninstall the old drivers and then put on your own it should take care of that. I'm finding with newer hardware and OSes you actually can get away with moving a hard drive to a completely foreign system and get it to boot, though the drivers and registry are so unoptimized that it will take forever to load each time at boot up. Clean installs are always a good idea if you have the time.
But otherwise, I don't think there will be any huge incompatibilities with your idea.