Bring a tape recorder to your next practice and make a very crude recording then have a post-mortem over a couple of beers after. Tell him what he is doing wrong and how to improve it. If he has any sense he will treat it as a learning experience and you will end up with a better guitarist, though you will have to do your best not to be patronizing !
If that doesn't work, kick him out !
My
instructors page and
www.studiotrax.net for all things recording.
my toons Brought to you by Dr BadGAS