I imagine all of us have gone through this.
I think the best way to look at it is that your pre disposition to HPPD might, one day, turn out to be a massive positive... Imagine that you get better over the next few months. You will have had your warning and will hopefully heed it, grab your new found health and happiness and run with it. Your friend might spiral down.
Alot of my drug friends that seemed to be untouchable were far from it. Everyone got burnt in one way or another... Some became junkies, some ended up in prison, some developed other mental illnesses, 2 are dead now. Even after 20 years, I still look on HPPD as a partial blessing, those that became junkies suffered worse... and I was well on that path.
Good luck, Jay