Relationship problem-help. Helpless : (

My sister is 25 yrs and she was in a relationship with 23 yrs old guy who was her classmate. They were together from last 3 years nd then my sister comes to know that the guy is engaged to someone else. When we got to know this..the guy explained that he loves her nd not her fiancee and didn't tell her the truth as she wud have been hurt... she is very angry on that guy and also she tried breaking up but again started contacting him herself as she now says she can't forget him ... that she loved him... nd now she wants to het married to him... in last 3 months they have both been fighting and the guy even hit her in front of people on road and recently she also did hit him... this is wat is going on I need some guidance and help counselling for her as me and my parents tried explaining her to move on but she is just not able to do so ..We all are scared now as she is asking to stay with that guy nd is ready even to get married. she is ready to end her life she says.. we r helpless :(