A boy threatens follows my sister

As my younger sister is so much tensed and because of her tension..all day I only think of her that what's the best thing I can do for her...as this started a year back my sister is a school student and she travels with school bus from home to school..and school to home..so their was the conductor she likes my sister he was around 20 he constantly tries to talk with her ..he cracks jokes and whatnot to gain her attention but she tries at max to avoid but he was not looking for that..he searched for her on Facebook and send the request..as she was actually just of 14-15 and she too new to fb and excited one ..she accepted all the request that come at that time and of that conductor also..he started sending text to her talking..she also replies and soon he started to send love text type and talking personal things and telling about himself and getting an emotional attach as I have lost my dad..my two sis..only mother is there means making an emotional bond that he is too alone so she also started replying and eventually she also started talking to her and like to talk but what he do always that he tells her everything what bad happened in his life and she is going to be only the good thing and as he was a conductor in the bus they meet and talk their too and what he did that please come and meet me at this side of school or that side otherwise I will do something..I will cut myself or something or I will not eat food..that he forced her emotionally always and he listens to her..her family problems and that yaa ur family is very bad they always do..I am the only good for u..nd I know u love me and what not and always he do something or the other too that makes cut write the alphabet of her name with blade..or doesn't eat and she was all scared and afraid that if she says no nd if she says something he will do something and harm himself and they both decided that what all they have they talked will be between them only and she never told anyone but he told everything to his mother friends share the photos of her and everything they talked as the matter of pride that he has a girlfriend nd what not .. they were in so called relationship and he was of very low back ground..low mentality and what he do that call me or I am at ur colony entrance I will come inside or like that stuff..and she tells everything to me as she shares everybitt of his life to me..so I told him that what all these typs of boys really do and what they all think about is having a beautiful girl..nd using and showing to the world nothing more than that but she opposed that no it's not like that..so I explained and she decided to say to him that she wanted to be only friends and not to be called in relationship as she was then also in a thought that if she says that no contact and anything like that he will do something wrong and name will come of her that because of her he did something and his friends who are also of barest background only will come to him and boys family too that this girl did something to their boy and everything will be out..andmy family will boycott her..she was always in this feeling and afraid of loosing the family..so she said that to him but he think that she wanted to be friend and then again girlfriend they r just starting new but after sometime she cleared that she don't want anything and don't want to continue she want to study and all..and she want to finish everything than what all the boy started that he will die..and doo something his friends started calling messaging her that he is in fever he got an heart attack save him or hee is not eating anything he will die because of u tell him that u love her..he is drinking he us cutting hee is in hospital he will die we will not leave u if anything happens..all day every hour these typ of messages start coming he started calling to my mother's phone and threatening type and messages call never stop from last 3-4 months all day first some pictures that he is cutting himself and all blood vomiting and what not and sending curses that u left me because of u r rich I am poor and his friends threatening that if anything happen to our friend then we will bring his body on ur doorstep..and many things she is just fed up of all that..and this all shows his true colors too..they even message many time that he is in ICU doctor has said that only few hours left only u can help..or he is dead and then no wrong news or they send the pic that he is dead and after few days again messages started she has blocked from all but never stopped .. hee come to school bus stop and keep looking or infront of his colony gate..and messages that reply and he is just at the back of her and he is like that he had told that everyone his full locality his co-workers team type and given the photos of her..and he is someone like a criminal type no background and can do anything can kill nothing to worry or think ..so what can we do the best or all to stop and close this matter for full life .