Has he owned the car since new? Forfours can have limiters programmed, I don't remember if it's on one key or all of them. But i suppose the 'LIM' would light up in the dashboard.
Is he otherwise down on power? What is the 0-100 time? To only manage 150-160kph top speed the engine would have to develop 80hp at most, so he would be missing 30% of it's power, and 0-100 times would be ~12-13s instead of 9.8.