What is the religious ruling regarding a wife's oath to forbid her husband to herself until the Day of Judgment if he marries her friend, and he has indeed married her?

A wife prohibiting herself to her husband is not considered ẓihār; rather, it is an oath that necessitates expiation. It is impermissible for a wife to refrain from her husband or to prohibit herself to him, whether he marries another or not, because a man marrying a second wife is permissible and lawful. It is obligatory for the wife to repent to Allah, to offer expiation for her oath, and to treat her husband kindly and live with him righteously.