Is expiation obligatory, as well as concealing the girl and marrying her without the wife's knowledge, or is it obligatory not to marry a second wife, after the father forced the husband to swear on the Quran, knowing that illicit sexual relations occurred between the husband and the girl, and she became pregnant by him and then miscarried, and that the wife stipulated divorce for him to marry a second wife?

It is not permissible for a wife to stipulate to her husband that he divorce her if he marries another woman, because it is forbidden for her to seek divorce without a legitimate Shar'i reason. Her leaving his house without his permission makes her a disobedient wife (nashiz). The husband and the woman with whom he committed adultery must repent. If the woman repents, there is no objection to him marrying her if he is capable of being just between his two wives, and he must expiate his oath if he marries her.