Given the phrasing and the chapter title (and the cover picture of the girl in a memorial photo,) it seems like it might be a situation where the girl was "meant to die," and thus unraveling their current situation could result in her death resuming as planned.
Edit: Given that the girl has access to the boy's memory while the boy doesn't have access to the girl's, and the girl's body was in a coma while the boy's body was not, it might be a case of the girl's body still being braindead/comatose while the boy's spirit is possessing it. If the boy does start getting access to the girl's memories then that might be an indication that they could feasibly swap back without the girl dying.