Thanks for the elaborate explanation. You really know a lot about this particular and general subject. To my very limited knowledge, and as the guy told me, it was stress-related and the context of his life at that time really reaffirms this. I think alopecia areata, to some degree, can be triggered by stress, among other things I have no clue about. Well, he regrew everything, so it's all good. Comedy in hindsight, right?
I'm a bit of a weirdo. I generally gravitate to the subject of health and I used to be deep in various health communities (even fringe ones as they tend to be lots of fun) so it exposed me to people with different conditions. And I would read about their anecdotal experiences.
I just wanted to make a distinction between stressed-based hair shedding that thins out hair and patchy based autoimmune hair loss. Psychosocial stress is certainly a factor in Alopecia Areata. Oddly, my classmate in Grad School got it and she was a lifter (beacon of health in some people's eyes). She told me it weirdly happened after trying to do CrossFit.

It can disappear (like in the case of your friend) and never abate or re-emerge. I guess its case-by-case. Autoimmune conditions are confusing. I even found out I have a high genetic susceptibility to Celiac (gluten intolerance). How common is Celiac among Somalis? I only met one with the condition so far. One thing is for sure we're getting odd afflictions from a highly industrialized lifestyle we may never have had despite having genetic susceptibilities.
Genetic predisposition could in theory occur for generations without any noticeable impact to health until an individual is exposed to unfavourable environmental toxins that may trigger it. Not to mention we live in more sterile environments, are exposed to an aggregate of chemicals and our gut microbiome has less diverse bacteria than say Hunter gatherers like the Hadza of Tanzania. I came across the below abstract from this article: