Is assisting a Muslim friend in marrying a Hindu girl considered a sin?

It is not permissible for a Muslim man to marry a non-Muslim woman unless she is from the People of the Book (Jewish or Christian). Otherwise, his marriage is invalid and forbidden by Islamic law. The proof for this is the Almighty's saying: "This day [all] good foods have been made lawful for you. And the food of those who were given the Scripture is lawful for you, and your food is lawful for them. And [lawful in marriage are] the chaste women from among the believers and the chaste women from among those who were given the Scripture before you, when you have given them their due compensation, desiring chastity, not unlawful sexual intercourse or taking [secret] lovers."

It is not permissible to marry a Magian, Hindu, Communist, or idolatrous woman. The proof for this is the Almighty's saying: "And do not marry polytheistic women until they believe. And a believing slave woman is better than a polytheistic woman, even though she might please you."

Therefore, it is not permissible to assist in such a marriage. Rather, the suitor should be advised to invite her to Islam. If she embraces Islam, then he may marry her. But if she insists on her religion, then he should not marry her, for whoever gives up something for the sake of Allah, Allah will compensate him with something better.