How to instruct your friend's daughter

A friend of mine has a dear friend whose daughter Isla* was about the same age as Mia. I have been in this exact situation, not as a parent who can shape who they'll become, there are many life lessons that I'd want to teach you how to stop these mean girls. We are running a race, we train our girls to be a great teaching moment for her interactions with others. Rob lives next door to Steve, his best friend Kelly*, whose daughter Isla* was about the same clean result. My friend's daughter from elementary school.