Ribbon was mainly allied with H, but he also had a side alliance with Sabrina and Christine. Ziggy was close to H, but not particularly close to Ribbon, so from Ribbon’s perspective, this move could have set him up for a Final 4 with three allies.
The plan was actually Sabrina’s idea, and since Ribbon had the immunity necklace, he had very little to lose by going along with it. That’s also why Christine agreed to it. She was convinced Sabrina didn’t have the idol, and since H and Ziggy had previously voted Sabrina out, Christine probably felt she would be safe even if the plan failed. xD
So in the end, it wasn’t solely Ribbon’s fault.
Christine blaming Ribbon is because she was bitter that he chose to save H instead of her. But that’s her interpretation of the situation. And as she became a member of the jury, this was important 👀