A 33-YEAR-OLD man died of a heart attack after an argument with his mistress, who wanted to end their relationship, and a man posing as her new boyfriend, an inquest heard yesterday. A jury returned a verdict of death by misadventure on Mr Lau Chin-woon after hearing that he had a greatly enlarged heart. Coroner Mr Richard Day heard that Mr Lau, who was married with three children, had known Miss Chan Lai-shan since 1988. Miss Chan ran a piano tuition school in Aberdeen, where Mr Lau had an office. In August last year he was admitted to hospital due to his heart condition. About a month later, Miss Chan decided the relationship was over. She said he would not accept this, so she wrote to him. She saw Mr Lau briefly after he telephoned her onOctober 17 and he was highly emotional. Mr Lau left for dinner, but told her he would return. Meanwhile, she contacted a friend, Mr Yeung Yiu-shing. They agreed he would pose as her boyfriend. Mr Lau arrived at the music school that evening and begged Miss Chan's forgiveness. Mr Yeung put his hand on her shoulder, but Mr Lau tried to elbow him away. They pushed and pulled each other until Mr Yeung asked him to calm down. Mr Yeung suggested the three of them should go out together the next day, to see ''which way she went''. Mr Lau then said he would get a divorce, but Miss Chan told him to ''be good and go home''. Suddenly, Miss Chan said, Mr Lau fell to the ground. They tried to resuscitate him, but he was on arrival at hospital.