Top tip: when helping someone to reverse their vehicle into a small space - don't stand behind the car and beckon them with gusto until you think they're about to hit something and then show them your palm like some kind of traffic police.
(Instead, how about holding your arms apart to show them the actual size of the space and reduce the gap between your hands as they near the obstacle? That way they get immediate feedback and can see ahead of time how far they have left.)