Right, but legally if he was drunk and the engine was running, even if it was to keep him warm, he could have a problem.
I hope he gets an excellent lawyer and then does what he needs to do, so he does not to it again.
Back when I was a law student in the 1970s, I saw a case where four kids were sitting in a parked car passing around a joint. It was wintertime, and they had the engine running to keep the heater on. The kid sitting in the driver's seat was charged with "operating a motor vehicle under the influence of narcotics"; the other three were charged with simple possession.