I've seen a few cases where a wheel of a bike on a rear rack dragged on the pavement as the car backed over a low driveway cut. Bent rim. Then there was the time I sold a bike to a guy who put it on rear rack. The whole rack, bike and all, fell off going up the street, before he was even out of sight of the house. That's why you shouldn't help them fasten it on - if anything happens, it wasn't your fault.

In whitewater kayaking we tied kayaks onto the roof of cars and vans, sometimes lots of kayaks on one vehicle. I always tried to make sure the person tying knew his knots and rigged things in such a way that more than one fastening would have to fail before things started hitting the road. Which they sometimes did.