Jo hates entirely the fact of standing there, watching his shock blossom, and the hiss that nearly escapes her mouth when she tries to bite down to keep herself still and unflustered, only for the action to set off an explosion of pain, black and red spots across her eyes. Her eyes water and it causes her to blink and her refocus on his face, the hands up toward her, like she might shy away like a scared animal (and how much she hates that, too), a little warily.

"How?" Except she already knows that doesn't she?
She's read about it, about Castiel healing Dean or Sam.

But she hadn't thought about it. Perhaps, if Cas was here. But his son?