In-law harrasment

Hi, This is about my cousin sister, she has been married for 5 months(feb 2015) now. She is the only child with some(very minor) mental issues form childhood. Now her inlaws are not letting her to talk to the parents and not letting them to meet with her, she lives in Delhi and parents leave in Nagpur. Parents sensed fishy about her behavior in the early stage and ask her to stay with them for 15 days to know her situation at the Inlaws place. But that did not work she did not utter a word but was constantly under her inlaws influence over the phone. Then very recently on her birthday she was called by all the family member and near relatives for birthday wish.... And in one such call she forgot to disconnect the phone.... then suddenly there was very loud shouting voice hear on the other end... it was non other then the father in law ... scolding the girl for dowry and insulting the parents and all non-sense stuff... and we could not hear the girls single word against him. It is believed that generally she gets scared by loud noise and domestic fights. She never complained about any incident happening at her house only form the phone incident all came out loud and clear. Now please I request some body to help the parents with some suggestion what has to be done to save the child. She just turned 23 yrs. Should they go to police or NGO or legally. All your immediate response are very much appreciated.