If your friend had rammed some rags up the exhaust tail pipe, the back pressure would have stalled the engine. But I know what you mean - you tend to panic when things spiral downwards.....
We had a rather strange (err unusual) guy working for our company 3 or 4 years ago. He drove a small Suzuki 4 wheel drive thingie to / from work. He was always carrying things - things like like raincoats, lunch boxes, news papers, plastic bags, etc.
He left the Suzuki as usual and went into the office to start the day - when he re-appeared 8 hours later to go home, he found that he had forgotten to switch off the engine
The poor car had been sat idling in the parking lot from 0700 hrs to 1600 hrs. He just shrugged his shoulder and drove home....
Mind you, he was the kind of guy who believed that the car was serviced each time you filled up with fuel!! And you don't need to change modern oils either!!
As I said, he was a little strange.