Hallucinations are a serious sign of altitude sickness, specifically high-altitude cerebral edema (HACE), where the brain swells due to a lack of oxygen, according to the Cleveland Clinic.
HACE affects less than 1% of people who reach an altitude of 13,000 to 18,000 feet above sea level.
This is what they didn’t account for. It’s already a rare condition, and one that only a few people will ever find themselves in a situation to discover they have it. Poor guy. I hope he recovers okay.