This page of the guide lists all ways in which Abigail can die. Remember that the game features the "death rewind" option, which you can use if you die by accident.
Considering the content of this page, a spoiler warning is in order. Below, you will read about important choices and situations that happen during The Quarry's storyline. For obvious reasons, we had no way of avoiding spoilers, but we tried to describe the storyline as vaguely as possible, and only point to specific situations.
In chapter 6, Abigail can be killed by a transformed Nick when they are in a room together. Right before that happens, the boy will act weird, and you will have the option to shoot him.
To save Abigail in this situation you must shoot Nick when the girl picks up the gun. If you don't, the transformed boy will immediately jump at her. If he's been shot, he'll jump out the window right away.