If he was in a closed building with the motor running then he could get carbon monoxide poisoning.

Was the wife or a family member home??
Suggest they take him in the house?
And take away his keys....
I would be concerned he might wake up enough to try to drive
Or call police to check on him...it might get him on the road to some help or a wake up call anyway...
I don't know if sitting in a running car & being drunk as a skunk is legal or illegal- cops might think it is illegal as person might try to drive.