Accident with a stranger’s car!

Hello dear forum members, yesterday had a small accident with a car from a buddy. Briefly to the situation: wanted to drive by car from my buddy from his home short to me, because my car was parked, took the key and am gone. So far so well, the way home leads me over a short piece of highway. I turned off from the highway and at a junction left, through the wet road and a little too much at the gas came to me the rear and could not more capture and then stood by 180° in the field, nothing happened to me because of the low speed and on the car externally as far as you can judge only the bumper side off and the rear insert went down on the exhaust, as well as a few scratches on the side skirt. Whether something has been warped will still turn out. My question is now, because of the unauthorized abstention of the key pays my PHV or has to pay his insurance for it. Theoretically, the insurance or the driver group so determined that I should also have driven. For your help I would be very grateful? LG Markus