The therapist failed to warn the victim or her family, and the patient made good on his threat. Failure to issue such a warning can mean civil, but not criminal, liability. The killer's best friend, a student at the University of California Berkeley, knew about the killing but didn't tell anyone. The law imposes a misdemeanor for failing to notify police about a violent or sexual assault on a child under 14. Virtually all the good Samaritan laws nationwide relate to a citizen's obligations after the crime has occurred or during its commission.
None addresses the situation of a person who fails to do anything before the fact.
The better thing to do is to invest the time to have teachers and parents talk to these kids about what's at stake here.