don't blame the pot,.. some of my most successful friends are weed heads! you seem to be hinting that pot & music are enemies of some sort. lol,... i got this image of your talented, creative friend hitting a joint, then standing there drueling while plucking one note through the entire song. (if his weed is that strong, i'll be his friend)
some people just aren't motivated. tell him to take a break and maybe it would help HIM. the alcohol might be more at fault, yet you seem to have emphasized the pot as the reason. in my experience, pot is a great motivator, but can also be a great relaxant. YMMV
Pot can kill your motivation. If you disagree now, come back 20 years after you smoke your last joint.