Late in the day by Tessa Hadley

Alex and Christine and Zach and Lydia have been close friends since they first met in their twenties. Thirty years later Alex and Christine are spending a leisurely summer evening at home when they receive a call from a distraught Lydia. Zach is dead. In the wake of this profound loss, the three friends find themselves unmoored; all agree that Zach was the sanest and kindest of them all, the irreplaceable one they couldn't afford to lose. Inconsolable, Lydia moves in with Alex and Christine. But instead of loss bringing them closer, the three of them find over the following months that it warps their relationships, as old entanglements and grievances rise from the past, and love and sorrow give way to anger and bitterness.