It’s true she can’t get him committed, but she could be more careful with the guns. Everyone should have a 2nd amendment right, but people need to respect the responsibilities that come with the right. If their kid is screwed up they need to be extra careful with the key/combo. It may come out that the mother was careful. Or it may be that she wasn’t, or worse.
She may have had the guns to protect herself against him.
As for his getting at them, possibly she didn’t keep them safely enough, or possibly he simply knew how to get at them. Very often people who seem totally nuts are very crafty. I’m sure he had planned all this in advance.