There have been similar incidents caused by the stroboscopic effects of props/rotor blades passing through sunlight, a syndrome which I think is called 'flicker vertigo'. It is similar to watching stroboscopic effects on tv and thus a warning is given to viewers, the end product is a seizure-like state with varying depths.
Perhaps your friend may be susceptible to this effect? I'm sure someone with more knowledge of the comparative signs of this and 'panic' will be able to comment.
