I agree I just find it funny he still thinks that if ur attention is on Layla and Elena that entire trip
The real answer is that there was meant to more variables to that scene other than whether John or Val does the rescuing and Zach was only facilitating a meeting between MC and Ethan rather than straight up betraying him, but Nyx decided not to go forward with a planned plot so that forced it to be a betrayal on all paths.
But to play devil's advocate here's a potential offscreen conversation:
Ethan: How's it going with that girl you like? Did you make your move at the camp.
Zach: I thought it was going well but she friendzoned me.
Ethan: I thought she was into you, what happened?
Zach: I don't know, before the camp I was sure she'd go out with me.
Ethan: Did anything happen on the way to the camp with her?
Zach: Well she was sat next to MC and I asked him to swap seats so I could sit next to her.
Ethan: Did he swap seats?
Zach: No he refused. OR Yes, but she refused the switch and insisted we switch back.
Ethan: Sounds like MC was looking to make a move himself OR Sounds like MC had already started making moves himself.
Zach: But he's already flirting with Layla and Elena.
Ethan: Some guys are just greedy and want to all the girls at their feet, I think he said something to make her think twice about you, it's the only reason she'd change her mind about you.
Zach: Shit, you're right.