To an extent you friend's right. Bad practice can hurt. Why practice bad mechanics? Makes no sense.
Some days I'm just off - so I just put down the cue - I'm just not in the mood - I just won't practice bad - I just go do something else. Some days I'll focus on what's off - twisting my wrist, moving my shoulder, whatever - and WORK on correcting it. Other days the balls just seem to drop and I'm psyched for my next match. It's always intrigued me why sometime you're on and sometimes off. I wish I knew. But, back to the topic, I think it hurts to mindlessly practice bad. If you don't have some purpose, focus and desire to WORK on what you're doing hang it up for the day.
If your friend's not motivated enough to WORK - well, he's not gonna get better. It's his call.