"The paper stated that the driver of the lorry was helping the police in their investigation" means that the driver is cooperating with the police and providing information to assist their investigation.

When a news report says someone is "helping police with their inquiries" or "helping in their investigation," it is a polite way of saying that the person is being questioned by the police in connection with an incident or crime. It doesn't necessarily mean the person is under arrest, but they are providing information that the police believe is relevant to their investigation.