Uh, I've owned a lot of cars, four, six and eight cylinders. Any time one dropped a cylinder it was instantly obvious...the engine would shake on idle and it got progessively worse as the RPM's went up. You could hear it in the exhaust. On your "friends" car you would also have had more than one warning light and messages galore. I find it hard to belive he drove a car any distance at all with all that going on and a valve banging around loose in a cylinder.

Something else to this picture.