Why would a human demon be impossible? Assuming this conversation/exorcism were real, I see no problem.
Demons are fallen angels, thus a "human demon" would be human turned into an angel. But humans do not turn into angels, just like they don't turn into stones, or trees, or monkeys. All humans will stay humans until judgement day and in all eternity.
The idea that a human might turn into an angel or demon is superstition.