What is the solution for dealing with a friend who betrayed trust and exploited the wife's help in solving a personal problem to cross the line of decency?

A woman's conversation with a non-mahram (definition) man is permissible when there is a need, but scholars have been strict about it because it is a means to evil. A man's protective jealousy over his family is religiously مطلوب (required) and commendable when there is suspicion. If what you mentioned about your wife is confirmed, confront her, advise her, and warn her. Be diligent in raising your family upon religion and faith. Lenience in allowing non-mahram men to enter upon women can lead to attachment, so it is necessary to keep men's gatherings separate from women's gatherings.