Eddie doesn't know what's happening. He doesn't know where Alex went, because this isn't exactly her, not anymore, and she'd made him promise not to freak out. He isn't. He's sticking with her, scared as he is, and in the end, Eddie isn't the reason they get separated.

The monsters that come around the corner are the reason. They move fluidly, intelligently, and they separate Eddie from Alex before he even realizes it's happened.