Husband is seeking second marriage without consent of first w9fe

Dear sir, This is about my niece. She's been married 6 years ago. From the third month itself she started facing problems from her in-laws. Theirs is a joint family. She somehow managed the tauntings for few months but couldn't withstand any longer. She came to her mother's house and stayed for 5 months. There was no message or call from her husband. Later, he took it as an advantage saying she's not interested in him and threatened of giving talaaq. Few elders spoke to the couple and compromised them and left. Same thing repeated after a few days. It was mental and emotional torture. There was no physical torture. Meanwhile he demanded additional dowry which his brother-in-law given. It has been 5 months again she's away from her husband. Earlier he used to ask his wife for mutual divorce for which she didn't agree. Now he's threatening that he can do second marriage ( without any compensation or alimony) on the ground that his wife is away for so many months. Seeing the girls respect we couldn't approach police or courts. Please advise us what can be done in this regard