Christian.

Your friends car. I remember a Golf I had that had done the rounds. "Plopping" at idle I also noticed it was also quite bad when off throttle. There were no codes. All leads, cap, rotor, coil/ignitor, plugs, and vacuum lines had been replaced.
I found the closed throttle position switch would very momentarily go open then close again (you would only see it with the scope), the ECU had no idea if I was coming or going. Once that was rectified it ran perfect.

I'm not saying that is your problem, although possible, but it may be worth opening other avenues.

What colour were your spark plugs?
Any chance of getting a (once hot) ignition primary, injection, oxygen sensor, and that closed throttle switch (just to eliminate it) over a long time base at idle and same again but at 2000rpm.

Andy