A GANG who stripped a 15-year-old boy, held him down, whipped him and made him drink urine for allegedly betraying their ''big brother'' to the police were found guilty in the District Court yesterday. Judge Chua described the case as ''particularly nasty'' and found all five defendants guilty of false imprisonment and wounding Wong Chiu-wah with intent. Kao Tai-man, 19, a construction site worker; Jiang Siu-fu, 20, Chan Kin-kwok, 14, both transport workers; Mok Man-chui, 28, a garage worker; and Leung Wai-man, 17, unemployed, will be sentenced next Wednesday. Evidence called by Senior Crown Counsel Michael Holmes heard that on the evening of July 18 last year, the victim, who was a member of the Wo Shing Wo triad society, was stopped as he was walking in Tsuen Wan. He was escorted to a convenience store where three of the defendants were waiting for him. The men accused Wong of stealing heroin belonging to their triad ''big brother'' and betraying him to the police, who raided his flat and seized his heroin. The victim admitted he was a drug addict and, like the defendants, a member of the triad society. Mok accused him of being an informer and attacked him with a stool. The other two defendants punched him several times. Wong was taken to a room in the Ngai Yuen villa where the other defendants were waiting. He was again accused of being an informer, which he denied. He was stripped, held down and hit with a plastic pipe, water hose and leather belt and burned with cigarettes. Wong was forced to drink a mixture containing the urine of one of the defendants. Strips of tissue paper were put between his fingers and toes and burned. He was released at 2 am. His sister became alarmed when she saw his injuries and called the police. Wong was treated at Princess Margaret Hospital for three days. The judge found that the victim had given truthful evidence in the witness box and had not implicated the defendants falsely.