Maine

I had parked on a hill next to my friends new saab, and my girlfriend had opened the passenger door, but it is an older car and the heavy door got away from her. It left a dent on his car that is under both of our insurence claim points.there was 3 feet between the vihicles, just the door can open up larger than that. Is it my fault if both of the cars were parked on the hill? and do i have to pay for it?