Arika tries to stop Ruri from taking drugs further because it started to endanger her lives, but it turns out she still fine the day after and until now. Which makes Arika's panicked concern which lead to her second death feels unnecessary? Silly misunderstanding?
Still, for all its worth Chihiro wasn't trying to kill Arika, its accident and her actual crime is mostly keeping this and Ruri's crime a secret. Of course keeping this for years and lying to police's investigation can be a heavy crime too.
I wonder if Arika is alive? Realistically she shouldn't but then so does being buried alive after falling from stairs. I wonder if its gonna use classic trope where she washed away to the shore and fallen amnesia?