I'd say the OP was extremely generous because what the OP should have done was: 1)got the plate, 2) called the cops and let them handle it. Yes, they absolutely would have cared, and around here the driver would have been charged with OMVI because he: 1) was behind the wheel, 2) keys were in the ignition, 3) vehicle was running, and 4) an open container was in the vehicle and 5) upon examination by the officer would have noticed odor on their person. The passenger probably would have at least been hit with open container, as well. Doubtful this was their first of the evening.
How are you sure that the driver was over the limit? Maybe the driver wasn't drinking and only the passenger was drinking? Might be as simple as the boy was trying to have relations with the neighbor daughter?
Obviously, I'm not sure. I WAS ONE STATE AWAY! Seriously? That would be something that the PD would determine with a breathalyzer. The point is that if you are impaired and you are behind the wheel of a car with the keys in the ignition, more than likely you will be charged with OMVI. The cops are not going to assume that United Parcel Service dropped the car there, they will assume that the person behind the wheel drove it there.
I offer no comment regarding the possible carnal invasion you note. You would need to inquire with the young lad who was driving as to the honor of his intentions.