dancingalone
Grandmaster
As I recall, your friend practiced Chang Hon, or the Chang Hon forms at least. ITF is an organization. The actual system, I believe, is called the Chang Hon system. So while it may not be his system, it is related and still within the same art.
This is one of those areas, be it your friend, or anyone else, where a long time practitioner can either seek outside of his own art or dive deeper into it. While I do not see the dominance of sport going anywhere or lessening, taekwondo is still young enough that an advanced practitioner can still clearly see what taekwondo was before the emergence of sport dominance and that style of taekwondo within his or her own school.
But that takes a lot of work, connecting with people ,and seeking out source material, an undertaking that not everyone has the time or energy to do. Which is why I had said earlier that I understand the appeal of adding hapkido; you are guaranteed to have hoshinsul in a hapkido school, so it is easier to simply find a hapkido school than it is to hunt through TKD schools and see who has authentic SD and who just has filler.
He's sort of my second cousin if you go through our respective TKD lineages. We did study the Choi forms, but I wouldn't characterize is as studying the Chang Hon system. To me, that would mean we followed General Choi's syllabus as outlined in his Encyclopedia as well as his later teachings not documented in the books. We're from the Jhoon Rhee Texas TKD line.
I agree with your reasoning on why hapkido is an appealing avenue for TKDists. I guess it's probably up to discussion which dovetails better: karate bunkai, judo, or hapkido. And the answer likely depends on what kind of TKD you practice.