Two years ago, when the billionaire Hollywood mogul Sumner Redstone passed away, his granddaughter Keryn got a text message from an old acquaintance expressing sympathy for her loss. Keryn, 40, who was among the beneficiaries of her grandfather’s fortune after an estate battle with two of Sumner’s former live-in girlfriends, did not respond right away. But a few months later, Keryn texted back her old buddy, Scott Nathan, and the two began chatting. That’s where their stories diverge. The dispute, now in court, represents a strange aftermath to the legal drama featured in James B. Stewart and Rachel Abrams’s new book about the Redstone family and their media legacy.