What is the punishment for a wife who betrays her husband and agrees with another man to marry him while she is still married, if what she did is not discovered in this world?

A wife's betrayal of her husband is a grave act of transgression (fisq), as it infringes upon his right to protect his honor and his bed, and it is a neglect of her duty to Allah. Allah, the Exalted, says: (And whoever disobeys Allah and His Messenger and transgresses His limits - He will admit him to Fire, abiding eternally therein, and he will have a humiliating punishment). Her sin is immense, and her burden is heavy, and she deserves the wrath and indignation of her Lord, unless His mercy overtakes her and she repents. Her disobedience is among the gravest sins against Allah's right and the husband's right. This falls under ingratitude towards the spouse, which the Prophet, peace and blessings be upon him, mentioned as a reason for women being the majority of the inhabitants of Hell, when he said: "You frequently curse and are ungrateful to your husbands." There is no doubt that betraying one's husband is among the gravest forms of ingratitude towards the spouse.