Ruth’s attempted suicide. The way it’s narrated from her perspective as she sleeps and doesn’t understand what is going on makes it just that bit sadder.
The moment when Beatrice is shown the monument on the beach by Ruth and realizes that Heather was also here. She runs off in tears, saying "I hate her." and "I wish she was never born."
Hell, virtually any time Ruth is visited by Heather's spirit (whether real or imagined) counts, just for how subtle it is.
