@ Author,
1. Certain conditions have been laid down for a Hindu marriage to be valid.{S. 5 and S. 7 of The Hindu Marriage Act, 1955}
2. Certain conditions have been laid down for a Christian marriage to be valid. {S. 3 and S. 5 of Indian Christian Marriage Act; As a general rule every marriage under this Act shall be solemnised between the hours of six in the morning and seven in the evening and the place of marriage is a Church. Exceptions are, however, made in case where a special licence permitting a clergyman of the Church is granted. The Act of a person who solemnises a marriage beyond these hours and outside the Church in the absence of the witnesses without any special licence, has been made punishable under S. 69 of the Act. The punishment provided is upto three years imprisonment}
3. From your brief all that is known is that a Hindu girl marries a Christian boy at her home in presence of girl's side witnesses / relatives and no hint of 'which religious marriage ceremony' parties followed at home of girl. For argument sake a Hindu marriage can also take place at once Home, but for so to happen he needs to be converted into Hinduism just before marriage !
4. Based on para 3 doubt I say this marriage can be annulled for sake of paper decree (but which Marriage Act party will file suit for annulment ?) so that either party can be then free to re-marry as per their religious customs, traditions or be it under Special Marriage Act as in future if marrying intra-religion without need for 'conversion' as then the facts may be.
5. To me this looks more a case of contemporary allegation of ‘love jihad’ ! * Be that as it may sound, well, facts do stand out in Court if properly detailed at the onset. Hence, place more details for better advice OR seek Chamber discussion with an local advocate found via reference with factual details of parties over and above what is mentioned as brief here.
* https://indianexpress.com/article/india/india-others/christian-boy-hindu-girl-marriage-ends-in-separation/