You shouldn't pry into others' romantic affairs.
John sees his friend Bill kissing another woman that is clearly not Bill's wife, Susan.
John wants to tell someone what he just saw.
John calls his own wife and tells her that an anonymous friend is having an affair and tells her he wants it off his chest.
John calls Susan and tells her that he just saw her husband being a little too friendly with another woman.
Susan never finds out that Bill is a cheating piece of trash and lives happily ever after with him, in blissful ignorance.
Susan is devastated and is hurt as well as being totally distraught about her future now.