Arya Samaj Marriage

My cousin brother has married a girl after a 3 year long love relationship at an Arya samaj temple and obtained a marriage certificate from the same institution but not registered it with the marriage registrar. Both are Hindu and 28 years old. They have kept this marriage a secret from her parents. The girls parents have already fixed her marriage with someone else and the date of marriage is very near. They have also distributed invitation to their relatives. And the girl's parents are forcing her to marry this new guy of their choice or her father will die of health issues. Even though the girl loves my brother she is under pressure and is worried about his fathers health and respect in the society. And therefore she is insisting my brother to keep arya samaj marriage a secret and allow her to marry the other guy. She is also saying that she will not stand by him if he takes any legal step. My brother doesn't want her to marry anyone else. What should he do to stop this. Can he do anything legally. Or this marriage is not valid without court registration. Kindly advise.