Married to a us citizen and getting cheated and in turn he has filed a fraud case against me

Hello I am asking this on behalf of my sister. she got married to a us citizen and after she reached there she was mentally and psychologically abused. her husband never was emotionally available and said that he is gay and then denied and never made any physical relationship with her. Despite her efforts, he was always rude and non-available. In fact, she also found him talking to other women sexualy and kept chatting with them. she was facing all this abuse and even asked out for help from her sisters-in-law however they chose to be on their brother's side. now she moved away from them to our Aunt's house as she is mentally depressed and abused and asked for some time and isolation to deal with this. in which she also became a patient of migraine and thyroid. in turn the guy has now filed a case against my sister that she is fraud and only wanted the green card and that the marriage was never real. this is a very serious situation for us. we have been cheated. she doesn't even have enough finances to take any step. kindly help me with some advice . how can she save herslelf from these false allegations and fight for her rights.