Results: Should she or shouldn't she, that is the question!!

A man has been cheating on his wife (the whole time they have been together) as of this moment “the man” is cheating with one of his exes (from before they were together). The mistress is also married. “The man’s” wife knows he is cheating, she’s caught him before with different woman. Should she tell the “mistress’s” husband or should she stay quiet and walk away?? Keep in mind the “mistress’s” husband doesn’t like the man at all and it would cause a lot of drama!!