I have a friend he married a Christian girl in the mosque of the town but the woman is still married to another guy but she doesn't live with him, so is the marriage legal. Should I talk to the cleric who did the marriage?

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 is His slave and Messenger.

A married woman cannot marry a person until she gets divorced from her husband. Therefore, what this woman did is not considered a marriage in the Sharee’ah. This ruling applies to all women regardless of whether the woman is Muslim or from the People of the Book.

As long as a woman is married regardless of whether she lives with her husband or not, she cannot marry unless she gets divorced from her husband. On the other hand, a Muslim man should avoid marrying a woman from the People of the Book since dissoluteness, fornication and adultery are so widely spread among them that they do not give any respect to the marriage contract. Therefore, this person should hasten to repent, abandon this woman, and cut all his relations with her.

You have to exhort this person and disapprove of his marriage. The Imams and all the Muslims in that area should separate them if they are able to do so.

Allah knows best.