It's in the text. That's exactly what he expects of Ginty. He's cross that he's stuck in the cabin with Ginty who isn't able to help him come up with plans for biffing Foley one, so he mentally sticks her in the only role he can conceive for her, which is the nurturing one. He's a young teen, so it might be a bit much to expect him to understand Ginty's panic attacks, but given his own fear of heights (not to jump ahead too much), he's at least as culpable as Lawrie in not making allowances for other people's emotional states.

There are times when I have sympathy for Peter's predicaments but his wanting Ginty, who is dealing with the aftermath of a panic attack, to recognise his indirection isn't one of them.