Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh
If it happens by mutual consent of both man and woman, it is adultery. If it is done by force, it is regarded rape. If this husband repents his sin and show serious determination of this evil crime and signs of righteousness are shown to him, she can forgive him. If he insists on doing such an evil, does not attempt to reform himself or announce a sincere repentance, I suggest she claims divorce since he may be a cause of trial for her.
Almighty Allah knows best.
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