Apparently, the child making the threats is a well known bully with a history of similar behavior. Can she sue the school?
Edit: she is contacting the police and threatening to contact the media. I suggested she retain a lawyer. I'll circle back with updates as this unfolds.
Any threat of violence leaves the school wide open for liability. Tell your friend to document all letters sent to the school about the issue so the Principal can't say he didn't know.