i think the girl does not exist and the guy is only pretending her to be real so he does n...

An okay theory however, he’d only been in there for a few hours or so when he was put in there with the girl and didn’t really have the time for his mind to wither away so she most likely is in fact, real. He still has numerous “normal” parts of him as of this point in time and still has a hold of his rationality and sense so I believe the girl is real. If she had appeared later on when he’d started to slowly lose his mind then your theory might’ve been more accurate but as of now, she seems real.