In the book The Art of Racing in the Rain by Garth Stein, what way can a person change the course of their life through a single action?

In the book The Art of Racing in the Rain, there are several characters: Enzo, a dog who serves as the narrator; Denny, Enzo's owner; Eve, the threat turned friend; Zoe, the child that restores purpose; and Annika, the not-so-nice plotting teen. There are other characters, but I will focus on Annika to answer the question being posed.
As the story unfolds from Enzo’s perspective, the world is seen from the view of a dog who believes a human body is his next destination. His life is a combination of observations, choices, and life-changing decisions. Enzo guides the reader through the life of his best friend and companion, Denny.
While Denny does his best to live out his life, he does not always get things right. Denny struggles to reach his goals, and things become more complicated when he ends up as the object of a young girl's desires. Annika, a teen who is attracted to Denny, is determined to get her way and manipulates events to land her in Denny's presence. When things do not go as planned, Annika makes a decision that will create an emotional storm for Denny and those around him.
Annika makes the decision to tell a deplorable lie as a form of revenge for being rejected by Denny. Perhaps her age blinds her to the full scope of the damage her choice will deliver, or perhaps the pain of rejection blurs the reality of the consequence that is to result when the last domino falls into place. For Denny, a spiteful lie turns an already complicated life into a nightmarish reality. As he enters into a custody battle of epic proportions, battling a lie will prove to be extremely tasking.
Although Annika has an opportunity to change things prior to the trial, she chooses to commit to her lie. One choice will not only alter the life of another but will alter her life as well. She will have to live with the consequences of her decision. For Denny, the truth prevails, and all ends as it should. However, it does not negate the fact that choices always affect and impact more than one person. Someone is always affected by a person's choices, even if that someone is simply the person themselves.