What is the ruling on a man remaining with his treacherous wife who betrays him with another, while he knows of her betrayal and fears contracting a disease due to intercourse with her?

It is not permissible to accuse a woman of indecency except with clear evidence, such as her confession, or the testimony of four witnesses, or the husband seeing her [committing the act]. In such a case, he must divorce her, due to the obligation of divorcing an unchaste woman. A husband's silence regarding his wife's immorality is a grave sin and a form of cuckoldry (dayāthah), and the children remaining with such a mother is an even greater loss. If the husband insists on maintaining this situation, he is a transgressor (fāsiq), and his transgression permits the wife to seek divorce.