I'm now married to a person who was already married to an American woman. He went to the mosque to (establish the marriage contract) but when he divorced her, he did all the paper write in the civil court. He didn't divorce her at mosque, as he did at the time of marriage get married. I want to know if this woman still his wife. What should he do?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

A woman is considered a divorcee immediately after her husband has pronounced the words of divorce whether they are pronounced in a mosque, court or elsewhere. Similarly, she is considered divorced if she gets divorce from an Islamic court due to the harms she was facing.
However, if the matter is limited only to the legal proceedings of a civil court and yet the husband did not pronounce the words of divorce, it is not considered as divorce.
However, if he wants to divorce her and he is obliged to conduct it in a civil court then he should do so and pronounce the words of divorce; by doing so she becomes divorced.
Allah knows best.