What one should do if girl is blackmailing the boy when he said of breaking up the relationship

My friend had a relationship with a girl for 4 months. The girl was after him everytime. So he also started talking with the girl. They had consensus sex also. The boy came to realize that it's not good to be in relationship where the girl wants the relationship for timepass. He told the girl that he didn't want this relationship and let's break up. Let's be friends. He even said to girl that he had girlfriend before. So better not to continue the relationship.Everything was ok. After few days, the girl contacted his previous girlfriend throughout social media. Msged his ex girlfriend about her relationship with boy. Blamming her that it is her for which he has broken up. The ex girlfriend said the girl to stay with boy. Leave her alone. But she continues to threatens ex girlfriend through social media, even contacted ex girlfriend brother. The girl started blackmailing the boy that she is going to defame the boy in his working area. She will inform his parents and his society also. She will destroy his reputation. The boy told the girl that again and again that he can't continue the relationship. She said that she would expose his innocenceinnocence. She will make him cry. She had contacted his office head. She even contacted another two far relatives of the boy to tell the story. She would stop doing all these if boy continues to be in relationship. The girl used to screenshot and sent the boy what she is doing. Whom she has messaged. She is threatening to defame him.