Ouch.
I expected this when she woke up in the bathtub and didn't know why, but that was still unexpectedly brutal.
I actually feel the most sorry for Hiro, oddly enough. I think her feelings were real in high school, but they started drifting apart after she moved, as happens. Long distance relationships are hard. They had fallen out of contact for awhile, and Hiro probably thought they were over. At the very least, they are to her, though she still thinks favorably of Chiharu. She seems genuinely excited to see her at first, but switches to awkward and distant as soon as she realizes that she still considers them an item. She might even have moved on to dating Mao, but either way, she doesn't know how to handle it and responds poorly.
But then, this girl goes back home and accidentally drowns herself. I'm pretty sure the ocean promise was a dream (she seems to wonder about it in her drowsy stupor,) so all Hiro knows is she left Chiharu heartbroken, Chiharu went home, took sleeping pills, and got in the bathtub.
We see Chiharu's POV and the promise-dream, so we can be fairly sure she wasn't killing herself, but Hiro has no such reassurance. She must spend every day wondering, was it on purpose? Was it her fault? If she'd broken up properly sooner, or if she'd just behaved differently in that moment, would Chiharu still be alive? (Chiharu's mom is probably going through something similar.) That is some serious guilt to carry around.