the best course of action would be for her to recuse herself from the case.
For me, this is the only correct course of action. It is what she must do. Also, she must not mention the fact that he is being investigated to her husband. To do so would be not only a breach of "professional ethics" but, in most jurisdictions, a criminal offence, often called "misdemeanour in a public office". In Britain an immediate prison sentence would follow a conviction for this. Worse, if the revelation allowed the husband (if he was guilty) to destroy evidence she might be guilty of conspiracy.
However, if the participants in this scenario lived in one of the many countries where corruption is a way of life, maybe she could just tip him off and collect a bribe from the likely powerful and rich people he is associated with. She could even find herself with a promotion.
Is this a real life situation, or a hypothetical one?