HIPAA's Privacy Rule permits use and disclosure of protected health information, without an individual’s authorization or permission, for 12 national priority purposes:
-When required by law
-Public health activities
-Victims of abuse or neglect or domestic violence
-Health oversight activities
-Judicial and administrative proceedings
-Law enforcement
-Functions (such as identification) concerning deceased persons
-Cadaveric organ, eye, or tissue donation
-Research, under certain conditions
-To prevent or lessen a serious threat to health or safety
-Essential government functions
-Workers compensation
By my count, a disclosure to prevent a legal sale of weaponry to a violent patient would be easily covered by 6 of these 12 exemptions and arguably 9.