It looks like the parents just let their kid take his own car to their cottage. The Kid was dumb enough to leave the house without locking up or kicking out his friends. I can see it now: Oh please, let us finish up our game then we will leave! You can trust us. We'll remember to lock the door.
The real problem was handing over all that property to the son alone and also the son letting his friends have a blow out. And neighbors who did not care. Overall a recipe for disaster.
I posted this before ...when I heard our son was invited to a barely supervised hot tub party (co-ed), the veto sign was lit and stayed lit. That way nothing could possibly happen. Some of his classmates on another occasion decided to steal a car. So they were all involved in a felony. It sounded like fun at the moment.