I mean it's clear that any visuals are made up by her while at least some textual thoughts are his (textual thoughts accompanying the images she made up might be what she thinks that he'd be imagining).
For example, he may have imagined her wearing something hot, but any images of that including what she'd say in that situation could be 100% made up by her. Another recurring example is that she's naked in a bath tub while something happens, which is 100% what she thinks he's imagining, but based on absolutely nothing except her perceived image of him.