I'm with Terry, living your life ruled by 'what ifs' is not a life well lived.
My sarcastic-to-a-fault mind immediately read the trigger of the incident and thought "Well, if you were hiking alone you wouldn't have turned around to look for the friend, and never brushed the boulder. Accident avoided."
I know, it's flippant and insensitive, but in my mind, it illustrates the same point: Everything you do causes ripples in what might or might not happen in the future. Avoid unnecessary risks, but realize that you can't control everything, even sitting on your couch. Go have fun.
To play Devil's Advocate: It's entirely possible that if hiking alone, she might have never touched the boulder, made the loop an hour faster and been just in time to get smeared by a semi on the way home.