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You will need dates and times. You don't need to know the name of the actual dog owner, just the name and address of the neighbor where the dog was visiting. In most jurisdictions, they can be liable if they allowed the dog onto their property to roam. For liability purposes, the law normally defines the owner of an animal as the actual owner, a person that has taken custody (like a dog walker, vet or groomer) or a person that knowingly allows the the dog on their property. It may be a gray area of who the court will consider more liable, the girlfriend or the neighbors. If the girlfriend is a minor, it will probably be put more on the neighbor. If you let someone visit with their dog, you do assume some or all of the liability for its actions.
Just tell the police what you know: The date, the time, the names of any witnesses, what happened and the details of the conversation with the neighbor. If you don't have the neighbor's name and/or address, just decribe them and the property.