Not to be a dick, but you sound like you shouldn't give advice to your friend if you're asking these kind of questions.
Firstly, CPUs come in different sockets. You can't just put a CPU in any motherboard, they have to have the same socket and be compatible.
And yes, if you install a new CPU/Motherboard combo, you'd have to reinstall Windows. It might work if you're lucky, but it has a very high chance of becoming unstable.