“However, what else could she have done?”
Getting rid of the guns is a start, when you have a mental case living with you.
She probably had the guns to protect herself against him (she bought her first gun just after she moved to CT with him after the divorce from his father). And while I doubt that she intentionally made the guns accessible to him, I think she may have been in denial about his problems and probably wasn’t careful enough.
However, a mental case like that - and it’s obvious even from his photographs that he was crazy - shouldn’t have been living outside of a locked ward. Yet even if she had acknowledged his problems, I doubt that she could have gotten him committed because that is almost impossible to do now.