As the driver does not have a license the adult would be considered by the law to be in charge of the car. There is an offence called 'drunk in charge of a motor vehicle', and I think they could be convicted of that, if they were stopped and the adult was tested and found to be over the limit.
Out of interest, I heard of a local man who was prosecuted for this offence when police found him asleep in the back of his car in a car park, sleeping off a bender rather than drive home! He had left the keys in the ignition, so the cops said he was, technically, in charge of the car.