When and how Theseus dies, whether Newt and Graves are already a couple or not, is up to author. Just, Graves watching Newt struggle with his grief and helping him through all five stages, and Newt understanding that Graves is hurting too, he lost his best friend! and comforting him in turn.

Warm/hopeful ending, please, with the two of them knowing this is what Theseus would have wanted for them (perhaps he left them a letter to be opened after his death?).