Married Life (2007) on Netflix

In 1949, the middle-aged executive Harry and his wife Pat Allen are the example of a happily married couple. One day, Harry invites his best friend, the bachelor Richard Langley, to have lunch with him and Harry tells him that he is in love with the widow Kay Nesbitt. However, he is afraid to ask the divorce to Pat that would have her heart broken. When Kay joins them for having lunch, Richard feels attracted by the sexy woman. Sooner Richard accidentally discovers that Pat has a love affair with an acquaintance, but he does not disclose the situation to Harry or Pat, otherwise he would not have any chance with Kay. Richard dates Kay in many occasions as a friend trying to convince her that Harry would never leave his wife. Meanwhile Harry blends Pat's antiacid with poison expecting to kill her and spare his wife from the humiliation of a divorce. But when Kay breaks with Harry, he realizes that he has just lost his mistress, his best friend and probably his wife.