First I posted to the (likely dup article) https://news.ycombinator.com/item?id=49965895#49966728. Wanted to post here, after reading more.

Is this an invasion of their privacy (reporting to police)? Yes, but possibly warranted?

Should a social worker have contacted them rather than the police? Probably, if for no other reason than to ask if they were serious about harming someone.

Difficult questions, I'm still undecided on whether it's OK to always ignore someone's rants, even if it may be (or they think it may be) a private diary.

Actually charging them with a felony seems pretty quick to accuse. (Maybe I missed a hint about how long the investigation took before the felongy charge?)