Update: The NTSB
recently posted the Probable Cause for this accident on their accident synopsis page:
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The student pilot’s poor judgment to take a multiengine airplane for which he did not have experience or permission to operate and depart into night instrument meteorological conditions, which resulted in a loss of airplane control and impact with terrain.
It appears that the mom's assertion that her son had been given a key to the airplane and permission to use it was, if not false, at least not supported by the preponderance of evidence. From what I can tell, it does not appear that there was ever any civil action against the airplane owner. I was worried that there might have been grounds for an "attractive nuisance" case that would have set an uncomfortable precedent.
The whole thing was a very sad episode, and I hope this report puts it to rest.
Thanks, Bob K.