While I agree she should have called his parents back, I honestly don't believe there's any more she could have done, otherwise. I certainly don't believe she's to blame for this heinous crime.
The defendant would not have agreed to voluntarily commit himself, and she didn't have enough "evidence" to prove that he was a danger to himself or others (she wasn't aware a notebook even existed back then & he lied to her about having specific plans). Without knowing those things, coupled with the fact that he wasn't acting incoherent, or psychotic, or out-of-touch with reality at the time, it's unlikely that a judge would have agreed to an involuntary confinement. Psychiatric patients say terrifying things and make empty threats about all kinds of things, all the time, to their doctors. But the majority of those patients never make a plan, act on it, or even mean it.
Every psych doctor on this planet has probably found themselves in a terrible Catch 22 at least once throughout their career. These doctors worry about being sued if they commit a patient who might not need to be committed. They worry about being sued or losing their license if they violate HIPPA laws if it wasn't absolutely necessary (which is a grossly objective decision to make---and will ultimately rely on the decision of a review board who doesn't even know their patient). They worry about making a poor judgment call--like trusting a patient who says he doesn't have a plan to carry out a verbal threat.
They're damned if they do and damned if they don't. Hindsight will always be 20/20, but those decisions are incredibly difficult to ascertain in the present tense. All they can do is use what they've learned through their education and experience, and follow the legal guidelines that are set in place. But they aren't mind readers or psychics... they're human and can't predict the future any more than anyone else can, and unfortunately, sociopaths are experts at making others see what they want them to see.
ETA: I posted this essay (lol) before I read the other posts on the subject... so I hope no one thinks I'm trying to carry on the convo after it's over or jumping down anyone's throat or anything like that... because I'm definitely not.