How to I confront my wife about running over my dear friend?

I can't post this on my main account as I consider myself too afraid to ask. When I first met my dear friend, we had a puppy that was very lovable, and very sweet. My friend loved playing with it, and would play fetch with her for hours on end. One day, my wife was bringing groceries in the car, and ended up hitting our puppy and killing it instantly. She's been very devastated and distraught for the past few weeks. I can't help but feel bad, I just want to know how to confront her on it.