There are two issues. (1) It may or may not be actually illegal to smoke on the balcony. This, I suspect is not true, as the balcony sounds as if it is part of a private home. (2) It may, though, breach a condition of the rental agreement or lease for occupants to smoke in public areas of the building, or on the balconies of individual apartments. This latter may be enforceable by other occupants via a management company or tenants association, with sanctions available to punish the perpetrator.