Need assistance to handle this mentally exhausting situation in relationship

I have a gf with whom i've been in a relationship for 9 months.(She broke up with her bf just before coming into relationship with me because he was living in USA and was not giving her time and after that she proposed me and I made sure that she had no feeling for him and accepted her proposal.) Everything was going well until his ex-boyfriend who was living in USA came here few days ago. And yesterday they met and that's when she told him about her new relationship with me. He was trying to convince her to come back in his life but she was sure that she doesn't want to look back. Of course he was totally despaired. ( He had last year asked her parents about getting married with her and her parents had denied about that.) He today at 2 AM in the morning came to her house and asked in front of her parents that is she sure about not being in relationship with him. And she said that yes she doesn't want to be in a relationship with him. And then he asked about me in front of her parents (of which they were of course not aware ). And she said I'm just a friend of her. And then after making sure that he was not going to be in a relationship with her again, he left. After this incident her parents are worried that he will not let their girl to marry with someone else.(And my parents doesn't even have any clue about all this happening in my life). All this she said to me today around 2:30 AM and now I feel like to beat that boy hard. I just want to beat him that's it. What should I do at this moment? Please guide me.