You should check if your building has the reserves and operating funds the spend money on legal action and how much trouble it'll take to recuperate the cost. If the owner never sells, the building is out of luck along with the funds wasted on violating the proprietary lease. Don't be penny wise pound foolish. Use the funds on improving the building instead unless you find his tenant obviously breaking more than house rules. You can try sending a warning letter first if the building retained a lawyer that includes sending Notice to Cure.