There are a few questions here
(1) Why the knowledge and consent of parents are relevant here? Does she want to get divorce also without the knowledge and help from her parents?
(2) What would be the expected reaction of her parents if they come to know of the marriage and the subsequent happenings? Will they accept her grudgingly or otherwise or will they abandon her?
(3) Where is she staying now? With the Christian, with her parents or some where else?
(4) If now she is staying with the so called husband, where will she go after divorce?
(5) The boy falsely converted to Hinduism? What is the meaning of this? Is it that he just told the girl that he has converted to Hinduism and she believed it? Or did he follow some procedure which is not recognized under the law, for conversion to Hinduism? What was the exact procedure followed by him for the conversion? Was the girl present during the conversion ceremony?
(6) Married according to Hindu custom? What is the meaning of this? What Hindu custom? Was the marriage registered with the Registrar of Assurances as a Hindu marriage?
(7) How long after the marriage did the girl come to know that he was an unconverted? Christian and what was her reaction then? Did she protest in any manner?
(8) You say the girl was converted to Christianity. Did she willingly agree for that? If not what is the proof?
(9) After her conversion or before conversion, was their marriage solemnized under Christian custom? Has the marriage been registered in the Church register?
(10) At the time of Hindu marriage did the “Christian husband” have a Hindu name?
(11) Did he disclose his Christian name after the marriage?
(12) Was the girl given a Christian name after conversion?
Whether there is a remedy and, if so, what remedy, will depend on the answer to these questions. Under the Hindu Marriage Act, 1955 conversion of one of the spouses to another religion can be a ground for divorce. Even if she had agreed for conversion, she is free to reconvert herself to Hinduism. That is her fundamental right. She can go to Arya Samaj and get herself reconverted to Hinduism. If she is in distress, she can go to a women’s NGO, or even to RSS for help. RSS will surely help her.