Not that I'm condoning this, but, take the keys to the other car away, and give them a headstart and I bet they'd figure it out precisely one commute's time away from their next shift at work.

I think part of the difficulty is people 'learning' to drive stick in a parking lot. That's good for 30 minutes, but you won't actually get a feel for it unless you properly drive around.

Honestly though, I think if someone is actually incapable of driving a manual transmission car, then they probably shouldn't have a license in the first place, it shows such a lack of fine motor control that it brings in to question their ability to manage other aspects of driving.