Not so sure about that.
Last year I cloned a genuine Windows 7 Professional to another PC. Like an idiot I forgot to use the loader on the cloned version (I was only fiddling about with another PC and intended to wipe it later) and it deactivated my other PC and blocked me out completely.
I got a message telling me I was running a stolen/cloned copy of Windows and that was that, ended up having to do a complete reinstall.
So I guess it depends how far he has gone. Once you get the above that's it, as you can't even get into Windows to attempt to reactivate it.
That's why I just use the loader now, even though my PCs are running genuine serials. Like all other software these days you try to be legal and honest and you get screwed, whilst the thieving gits don't get no DRM or crap put on their computers.