If He Had Been with Me by Laura Nowlin

I wasn't with Finn on that August night. But I should've been. It was raining, of course. And he and Sylvie were arguing as he drove down the slick road. No one ever says what they were arguing about. Other people think it's not important. They do not know there is another story. The story that lurks between the facts. What they do not know—the cause of the argument—is crucial.