I would put more fault on the girlfriend.

Strictly, the reversing manoueuvre should have been completed without impeding any other driver.

Were I the girlfriend, I would be very meek and co-operative with the other side.

If they get nasty, and are able to interest the police (no mean feat), there could be a charge of due care and attention.

Similar story happened to me some years ago. I was reversing out of a "herringbone" parking slot and, whilst actually stopped, brake lights showing, to have another look round, a driver in an old Passat came charging round the corner and smashed into my tailgate, which subsequently needed replacing. (Absolutely no discernible damage to the other car.)
Within seconds his entire clan had surrounded the car to hassle me and there was, of course, not an independent witness in sight! I filled in my own statement and sent it to the insurance company. The fact that it was dark and raining and that he had no lights or wipers on, and actually had his small daughter sitting on his lap and a further six people in the car cut no ice. It was an insurance loss against me and I'm still angry when I think back.
I always reverse into a parking space and drive out forwards these days, however inconvenient it is. Supermarket parking bays are never designed with the driver in mind and I've never managed to execute a perfect 90 degree turn, but that's life!