For one, she would not know. No one can know. The best she, or anyone can do, is to assess a situation... to predict risk factors. These factors can include everything from obsessive beliefs that incorporate violence. Fixating on an individual and stalking them. Sinking into suicidal ideation. Importantly, notable decompensation. Extreme behavioral changes that might suggest organicity, a psychotic break with reality, something of that nature. Even then, unless the behavior is so extreme as to raise concerns, nothing can really be done. Not even a 72 hour hold. The key here, would be extreme and noticeable changes that would raise valid concerns. Again, am not stating that he exhibited any of this. Simply challenging your belief that a person has to tell their therapist some plan to do something to be put on a 72 hour hold.