What is the ruling on insulting the wife's friend with words that touch upon honor, while continuing the relationship with him despite the wife's torment?

It is not permissible to insult or revile a Muslim, for "reviling a Muslim is فسوق (disobedience), and fighting him is disbelief," and "it is enough evil for a Muslim man to belittle his Muslim brother." If the insult involves slander, it carries a deterrent punishment. It must be noted that a woman's relationship with a man who is a stranger to her, under the guise of "her husband's friend," is forbidden and sinful, and the husband is not allowed to approve of it for her. She must cut off this relationship and repent to Allah Almighty.