"Simulation sickness" comes from the fact that when there's a disparity between your visual sense of motion and your inner-ear inertial sense of motion, your au

"Simulation sickness" comes from the fact that when there's a disparity between your visual sense of motion and your inner-ear inertial sense of motion, your autonomic nervous system thinks you may have been poisoned and induces vomiting to help get rid of the poison. Your friend having flown real planes probably makes him even more sensitive to sim sickness because his vestibular system has been trained on motion from actual flights, meaning it can perceive the mismatch in the sim much more acutely.