You're assuming she didn't take him seriously.
As you are assuming she didnt. She said she thought he would be saying the same thing the next day. You know people have threatened it to try to get attention or to try to get people to do what they want, a teen girl isnt responsible to know the difference here.
His parents and other friends who said he threatened it all the time, i guess they are responsible too, since he wasnt committed for threatening it all the time.
If she assisted it, so did they.
If she were there and saw him preparing and then said what she did, then you would have a case. She wasnt there and didnt know he was serious and there is no way on earth you can prove someones thinking. (That she knew he had it all set up and was actually doing it)
You have a partial cry wolf scenerio here, and thats not her fault.