cress dies before thorne does. 

When her eyes droop with the signs of years gone past, when her hair, once a lively gold, goes moon-pale. One day her battle wounds catch up to her. The tears in her gut finally taking their toll on her.

Thorne stays at her bedside the entire time. She goes slowly, peacefully, and forces out words through the worst of it all. She tells him that she's always loved him, even before she knew who he was. That when they'd met everything had fallen into place.
Most of the things she says are just emotional gibberish, but Thorne smiles through the loss of it all anyway, clasping her hands between his as the life in her eyes fades. The beautiful blue turns glassy, and he blankly wonders if this is what a dead ocean looks like.
For days after–weeks–he wanders the Rampion alone. He refuses–can't–keep traveling, can't keep going, because if he does it will seem as if he left a part of Crescent behind. His sweet, smart Crescent Moon.
The rest of his crew try to help. Try, meaning cleaning up around the Rampion and gazing at him with pained eyes. Sometimes they make a move to speak to him, but the urge falls as soon as it rises. He is in no mood to talk.
It's on one stormy evening when Thorne, after months, hears a knock at his door.
He hasn't heard that sound in a long time.
Thorne opens the door.
A faint smile on her face, almost hopeful, too hopeful, Cinder waits. She's holding a gaudy, colorful dress, and Thorne recognizes it as the butterfly dress. The dress.
She holds it out, and without a word, Thorne takes it from her, the delicate fabric sliding in his fingers.
He looks from it to her.
Then nods.
No words need to be spoken as Thorne turns to his cabin window. The sparkling streets of Artemisia greet him as he looks out, over it all. At Artemisia Lake. Down to the water below.
Cinder steps forward, beside him. She's silent for an agonizing pause, and when she does speak it's an echo of her voice. He can barely hear her lips move as she murmurs five simple words:
"It's time to let go."
He doesn't reply. She doesn't need one.
Holding the dress of Crescent Moon, the only girl who could break his heart, over the water, Thorne let go.
The memories fell.