I heard it more as them buying a gun for themselves (though the kid described it as "my new toy"), it was locked up at home in a manner sufficient for the required gun-safety/keeping-out-of-the-hands-of-an-opportunistic-burglar laws, but there is no part of that law preventing access by family members and... well, that happened.
And then when Pops first heard about a school shooting, he drove straight home to check if "his" gun was still where he had stored it (and did not find it).
Noting, this is an assemblage of various facts and maybe 'facts'. It seems to follow logically, no doubt when it comes to trial there'll be evidence that not all of this is as logically true as first thought[1]. Shall we just say that the optics are very bad, currently. Corrective lenses might be provided later.
[1] Yes, like it seems obvious that someone travelling across state lines and arriving with a gun must have brought the gun with him, and as he has no drivers' license he may have been given a lift by his mother... I imagine there'll be contraindicative statements against the above, as well, at some point. I trust to the court to sift the information given to the jury without too much prejudice (or credulity) creeping in. Right now we're at the whims of the bush-telegraph and I'm open to the possibility that there are not yet enough absolute truths to be known here.