I think this teacher should take the threat seriously and respond accordingly. An incident report or some kind of formal written notice should be submitted to the school admin as well as the school superintendent. She should also look into filing a police report and possibly taking legal action. This may be the only thing that will get the school/school district to act to keep her safe, and to get the family to get their son help. I'm not saying that the kid would act on the threat, but you never know...and it's better to be safe than sorry.
I have a 13 year old middle schooler who is 5'10 and 140lbs. He's never been threatening to anyone in any way, so I have no reason to believe that he'd ever hurt me, but given his size, I know that he certainly could if he wanted to.