Private parking is used by other people ! What to do ?

HI people, don’t know exactly if this fits in here, but find no other subforum. I spoke today with my neighbor, a nice pensioner. She told me that on her private parking lot someone else has parked already, although it is marked as private. I then meant to her, why don’t they let him tow away. She meant that she had informed herself about this. She must pay the towing costs first and if the other person does not know Is that true? What exactly does the legal situation look like? What can you do about it?