How to sort out such behaviour?

Actually there is problem occured from few days and the matter is my bro and a girl in a relationship for 6years and she went to state and he is in India so they are far distance relationship..they love each other so much and they r planning to marry soon once she will be back. In between bro.got friendship with other girl and she told him to meet her as she came from Germany so she told him to meet as she doesn't have friends in India so he gave company to her once twice thrice when she needs company of him. Then she said to him she is falling love with him and he refused clearly bcos he had already his fiancee after that the story begins...that girl was started bullying his girlfriend by sending messages from Facebook by telling she is pregnant of her boyfriend which was not true and told her she will make her life miserable. She used words like pathetic and bitch to his girlfriend and trying to make them break their relationship. She warned my bro.she will be bad as you can't imagine..what to do plz help!