You must beat a friend's highlight. These are an individual's personal bests from people in a particular zone. They are not to be confused with lap times or high scores on leaderboards for Skill events. The categories can include a combination of Ground, Water, Air with Drift, Jump, Speed.
To find these easily, go to your map and then press square to see filters. Go to the left once to get to Highlights. Select Scope and then select Friends. This will cause the map to only display highlights from friends and makes it easier to find one to beat. Choose your preference and then beat their personal best. Once done, the trophy should unlock.