Divorce for my sister when husband is not co-operating

The marriage of my sister-in-law was done more than 3 years ago through shaadi.com. Within few days of marriage she fell victim of teasing, mental and physical torture for bringing inadequate dowry. She was never accepted with love and dignity. Her husband works in USA and they now have been living separately for more than 3 years. We filed a case for cruelty and harassment against him and his family but the case has been dragging on for long, made more difficult due to him being a NRI. Now we don't want to pursue this case, and for future of my sister want to get a divorce. His family wont agree for mutual divorce easily and blackmail for further money. So we want to file one-sided divorce. 1) He was prosecuted in USA once for soliciting prostitutes once after marriage. 2) Living separately for more than 3 years. 3) He and his family always treated her cruelly. Now our only concern is to get divorce as soon as possible. What are our option and how long divorce will take.