Is a woman forbidden to her husband if he has anal intercourse with her? Is it obligatory to separate them? And what is the expiation for such an act?

Merely approaching one's wife anally does not render her unlawful to her husband, even though it is a grave transgression and a loathsome act. An exception to this is when it is repeated noticeably, becoming a habit, and the wife acquiesces or is unable to resist it. In such a case, they must be separated. There is no text that stipulates a permanent prohibition in this situation; rather, scholars are of the view that if they repent sincerely, they are not prevented from reconciling. The expiation for this act is sincere repentance, increasing in seeking forgiveness, and performing righteous deeds. Allah Almighty says: (But whoever repents after his wrongdoing and makes amends, verily, Allah will accept his repentance. Indeed, Allah is Forgiving, Merciful).