How will Allah separate the repentant fornicator from the unrepentant wife and the husband on the Day of Resurrection?

Whoever repents, Allah will accept his repentance, and he must completely sever his relationship with that woman. He does not need to seek forgiveness from her husband, as that would cause corruption and evil.

He should earnestly supplicate to Allah Almighty to make her husband pleased with him, and he should frequently seek forgiveness for him. It is hoped that Allah will take care of that injustice for him and not hold him accountable for it, because "the one who repents from a sin is like one who has no sin."

As for the woman, she bears her own burden if she does not repent, and she is subject to Allah's will if she dies upon Tawheed (monotheism).