Fraud, cheating and false promises of marriage.

My sister is in a relationship from 4 years. They were dating each other and my family knew about this. Slowly they took one step ahead but before that my sister always asked him to talk to family first and he said he knows his family. They will say yes to their marriage but now after spending so many years together the scenario has changed drastically. Two years ago her boyfriend's parents were looking for match for their son and he didn't take a firm step and at that time my sister had to go to the place where he lived and then he talked to his parents that he said no to that proposal. And then now his parents are still forcing him nd he agreed to talk to the girl. He didn't even tell my sister about all this. And when my sister got to know she felt cheated. Then she called him and asked him to talk directly about marriage. He said he will talk. My sister asked him to marry in court because after being in physical relationship she can't marry anyone else. She trusted him to that level. And then my sister talked to mother and asked her to call his parents and give the proposal of marriage. They denied clearly and said ask her to marry someone else. And after all this her boyfriend is not taking a stand for her. His parents are manipulating him. They even called my sister and my sister told everything to them but still they are not ready for marriage. They are trying their every way out to end this relationship. Her boyfriend blocked him from everywhere and not even responding to any calls. What to do now? We can't leave them like this. He made false promises. We have all the proofs of denial of marriage and photographs as well. Even common friends know about them nd can testify if needed.