Is a prayer valid behind an Imam who secludes himself with a woman who is a stranger to him, under the pretext that she is engaged to him and with the knowledge of her family, and how can he be guided?

The fiancée is a stranger to the suitor before the marriage contract; therefore, private seclusion with her is not permissible except in the presence of a mahram, and she must be modestly dressed. If the Imam secludes himself with his fiancée, he is committing a sin and should be advised. Prayer behind him is valid, but it is better to pray behind an Imam who is not known to commit sins, if another mosque is available.