No worries! The driving score of a trip is assigned to a user only if the app is connected to the car adapter during this journey. As a consequence, if your friend connects to your car adapter, he will retrieve his statistics in his profile without affecting yours. Otherwise, the trip won't be affected to any user.
Statistics related to your car (mileage, duration of use, etc.) will be recorded by our servers in order to keep a global vision of its maintenance.