[adrusi]

What the parent was describing isn't HPPD, which involves sensory disturbances, usually minor optical hallucinations, and is mostly limited to people who used psychedelics heavily over a long period. The parent probably experienced the disturbances of mood that psychedelics can cause in the days following a trip, got panicked that something was wrong, and then the anxiety perpetuated their destabalized state. Or something else, but not HPPD.

[goldenkey]

More likely, he experienced derealization or depersonalization. The shake in consciousness can be jarring in that, one realizes the weight of being the "one" behind the sensors, heuristics, and thinking. The ghost in the machine... Really, there are books written about this. The Wisdom of Insecurity by Allen Watts. Life is intrepid and scary because consciousness is a black box. Anxiety is just a symptom of being shown the unusual, being shaken to a core of meaning that defeats the usual humdrum of assumed knowledge.

https://en.m.wikipedia.org/wiki/Depersonalization