All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
If this woman is a chaste Kitaabi (Jewish or Christian), then his marriage to her is valid, as we have previously underlined in fatwa 81758. There is no doubt that it would have been better for him to marry a Muslim woman, but no one has the right to object to his marriage to a Kitaabi woman or to boycott those who attended the wedding merely because the man married a Kitaabi woman.
However, if she was not a Kitaabi (not Jewish or Christian), then his marriage to her is invalid and prohibited to start with. He must leave her, and he should be given advice kindly and wisely in this regard. Those who attended the wedding while knowing that such a marriage was invalid have committed a sin and are required to repent to Allah.
Maintaining ties of kinship is obligatory, and it is prohibited for the Muslim to cut off relations with his relatives. Forsaking the persistent sinner for a certain period of time is determined according to what serves his best interests. If it is beneficial to forsake him (and effective in terms of deterring him from committing the sin), it is allowable; otherwise, it is impermissible to forsake him. Hence, it is impermissible to forsake those who attended the wedding without knowing that it was invalid (assuming that it was indeed invalid), or those who repented from that, and also those whom forsaking would not be of any benefit in deterring them from sin.
In any case, this woman should be invited to embrace Islam and should be treated with kindness in order to soften her heart towards Islam and, hopefully, she would embrace Islam and the one who invites her to it would earn great rewards. Verily, whoever guides someone to do a good deed will receive equal rewards. Sahl As-Saa‘idi, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said to ‘Ali, may Allah be pleased with him, when he was sending him to Khaybar, "By Allah, if Allah guides aright even one person through you, that is better for you than to possess red camels (i.e. the most precious possession)." [Al-Bukhaari and Muslim]
Allah knows best.