He doesn't have to tell them.
He hates the thought, hates himself even as the words flash through his mind. But later—after River's left, after he's changed the mocking desktop theme—he clings to them as shreds of consolation for his arid desert.
He's spent years trying to plan how to tell them, but even in his imagination, it always ended with Amy's hazel eyes glaring at him, sharp as Rory's sword. He couldn't—he can't—yes, he'd love to go back and save her, but he can't, it's a fixed point, it's how they met, and if you want to blame someone, Amy, blame your daughter, she told me what comes right before it for me, and I had to let her go, I couldn't say anything, do you know how that feels, letting someone walk to her death to save you, but a you who is absolutely, completely, clueless?
His hearts pounded faster at that bit, every time. He'd settled on telling River to visit her parents before the Library—well, asking, he'd learned at Berlin that he couldn't make River do anything, and Utah had hammered that lesson home.
Now he won't have to tell them, can't tell them. Darillium will come and go, and he'll be the only one to know why their song brings tears to his eyes.