Answer
(Fatwa: 1258/777/H=1432)
Iman and Islam are among the conditions that are necessary for nikah to be valid and to take place. If it is a fact that he did nikah with a non-Muslim woman while she did not accept Islam, then the nikah was invalid and it was not lawful to have the spousal relation rather it was haram. If such person reforms himself after he was informed of the Islamic ruling, it is ok, otherwise it is not lawful to treat him with Islamic tolerance. However, there is no harm in making him understand with wisdom, insight and politeness.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband