Having worked in the mental health field, the main concern was that patients would hurt themselves,not others.I'm alittle surprised based on Adam's profile that she wasn't worried about him being suicidal. It is very rare that someone is homicidal,and usually there is violent indicators before that. I would guess Nancy was responsable with her guns,and like most of us parents,would never dream that our children would be capable of hurting someone.
Was it wise,as you said NO,looking back,but if he had no violent tendancies before it may not have occured to her that just because he had mental issues that he was dangerous.