Very difficult situation. If you know the parents directly or indirectly through other friends, you may want to reach out to them. However, I think it is also a good idea to notify authorities with the public information and after you meet with them and get a feel for their level of competence, decide whether they can be trusted to keep the personal information shared with your daughter confidential to avoid retaliation.
Seems like it may be a cry for help in some respects if he's sharing that he wants to harm himself and the parents should be made aware. The threats against others should be taken seriously, even if the methods laid out are ludicrous. I think in some cases, kids reach out on things like this to see if anyone is actually paying attention to them and taking them seriously. If so, hopefully help can be provided. But if not, it just reinforces that nobody (in their mind) cares about them and makes it easier to kill themselves and/or others.
My $0.02. Good luck, not an enviable position for your daughter or family.