Mental harrasment and stalking

Hi, This is regarding my a situation my sister is facing. She had a friend (A) who was always too clingy. She has known A from school and he was a depressed lonely kid, so she felt bad for him and didn't cut him off after they completed their schooling. A used to keep calling her all the time and she used to keep avoiding his call until she could not. He used to keep telling her don't go out with other friends and all that. And they she would explain that it is none of his business and when he would not understand stop talking to him. And then A will keep messaging her non stop and calling her repeatedly till she talks to him again. Throughout all this she treated him like a brother and always addressed him as such. My sister is in a serious relationship for the past 3 years with a family friend and A knew about this. My sister has also try to help A out of his depression but encouraging him to make more friends, talk to a girl he likes and all that. Recently, A moved to UK for higher studies. His phone calls started increasing after that. He wanted to talk to her daily. And when she finally had enough of this and avoided him completely, he started calling their mutual friends and asking them to get to her to talk to him. When she refused he threatened suicide. I got worried and contacted his brother. Explained the situation to him. My sister blocked this guy on all social media. We thought this was the end of it. But after a month A has started calling her from unknown numbers. My sister did not replying to any messages or calls and I informed the A's brother again. A has been keeping tabs through friends on where my sister is and the people she is meeting with. I'm worried and scared for my sister's safety. She is so scared. A has started become aggressive in his messages where earlier his tone was always apologetic. The only reason he is not physically stalking and harassing my sister is because he is in the UK aand not in India. Please advice on how to proceed with this situation.