Is speedometer adjustment legal in Austria?

Servus, sorry, if I demand here, because I don’t like such machinations either, but I would like to have my question clarified. It’s about my friend, who turns back tachos in Austria. He claims that this is legal in Austria. His customers come to Austria about 80 % from Germany to have this kind of “service”. Does anyone know about the laws? This should not be advertising, because I am an opponent of such machinations. But he does not want to let it go, because it is supposed to bring in a lot of money and is allegedly not forbidden in Austria. I would like him to stop doing this, but unfortunately he has no valid arguments to convince him to let it be. Due to his permanent stay in Austria we see ourselves very rarely. I live in Germany in the Stuttgart area, and always have to travel long distances to visit him. He does not want to go back to Stuttgart. Although he would immediately find a regulated job as a car mechatronics engineer. He even once had his own workshop (old petrol station with lifting platform), but he had to give it up because of environmental requirements, and therefore even went out of business again with debts. He actually has the snout full of Germany, and therefore went to Austria. I have my work in Stuttgart and can not move to Austria.