Is the wife's sitting with her husband - after he told her, "You are divorced if you did this act in the past," and she knows she did it before marriage and repented from it - considered adultery and forbidden for her, or is it permissible for her to sit with him and rely on Allah, given the impossibility of informing him, her family, or going to court?

A divorce conditioned on an event that occurred in the past is considered an immediate and binding divorce. If this was the first or second pronouncement of divorce, it is a revocable divorce (ṭalāq rajʿī), and the wife must remain in her home and not leave it throughout the waiting period (ʿiddah). If her husband has intercourse with her during the ʿiddah, it is considered a valid return (rajʿah) according to some scholars. However, if the ʿiddah expires, she does not become lawful for him except through a new marriage contract. If this was the third pronouncement of divorce, then she is forbidden to him and does not become lawful for him unless she marries another man, and she must strive for his separation. One should also increase in supplication (duʿāʾ).