I know when I had a 1.8t golf that was misfiring it did not trip the CEL. There is a limit, and it it misfires more than a certain number of times in a given interval, then the CEL will turn on. Not sure if his car would be the same way.

 

All newer Audi's and the VW Passats will CEL almost instantly, plus with an S4 it would buck like mad and would most likely go into limp mode.

 

My golf was an 01, his Audi is an 04. Like I said, their is a tolerance built in before the CEL will trip. I had the car hooked up to a vag and watched the misfires accumulate, but the CEL didn't trip until there were several in a short period of time. If he really wants to know if it is misfiring, datalog the car.