You shouldn't ruin other people's relationships.
Dave is at the bar when he witnesses his friend Tony talking to a woman that's not Tony's wife.
Dave wants to tell someone what he just saw.
Dave calls his own wife and tells her that he's seeing his friend Tony talk to a strange woman.
Dave calls Tony's wife and tells her that he saw Tony with another strange woman.
Tony goes home as normal and continues on with life, without interruption.
Tony is furious when he learn Dave told his wife as it leads to a huge fight between them.