Hitting car parked on private property - who's responsible?

Hi there

Can anyone give me some advice? I have a shared drive with my neighbour. Tonight my partner's truck was parked on our side of the shared drive, but probably about 2ft was over the neighbour's side of the property.

She wasn't looking and reversed into his truck, doing quite a lot of damage to her car. She is saying that he is responsible because his truck was on her property and that he should pay for the damage to her car. Is this right? His truck was parked up with no-one in it.

Any advice gratefully received.

Thanks