To save Nick, you'll need to make one specific decision in Chapter 8. His first and only possible death can take place here, and it doesn't matter all the previous choices you have made through the whole story at this point. For example, he won't die if you shoot him as Abigail in Chapter 6.
However, you should shoot him there because he'll kill Abi if not. And while you're trying to save Nick, you should also save all the rest of the counselors to get the best ending and the Rough Night achievement.
Moving to Chapter 8, here you'll play as Ryan and Laura for a few scenes. At the end of this chapter, they will find some electrified cages. Inside the one on the left will be Nick in werewolf form. The other one will have Jacob, if he has survived up to this point.
After Ryan speaks to Emma about the place, she will aim her shotgun at the werewolf (the characters don't know that it is Nick). Here will appear a brief QTE in which you have the chance to interrupt Laura or let her shoot the beast. You must stop her, and she will leave the werewolf alone.
In this scene, if Jacob is alive, you'll have the chance to free him by solving a puzzle. This is not necessary to save Nick, but you'll avoid him killing Jacob in the next chapter. Here's a guide with the solution to the puzzle.
It doesn't matter what you do in Chapter 9 or Chapter 10. Nick is already saved and he'll remain alive for the rest of the story.
Nevertheless, you might want to lift his curse and restore his human form. To do so, you have to kill Silas, the White Wolf, at the end of The Quarry. This event will lift the curse and most of your infected characters will be healed.
To kill Silas, you need to keep Laura alive and make her work with Travis. They will find Silas and you'll be able to shoot him. After this, and if the rest of the counselors are alive, you'll get the best possible ending in the game.
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