Sounds like you're going through a rough time. I can relate. I've lost a few family members lately including my father. It's not easy. I've also dealt with a lot of alcoholics and drug abusers in my life. I don't know you well enough to know exactly what you personally are going through with your friend but if you're questioning yourself, please try not to. Addiction is just as cruel to the people around the addict as it is to the addict, maybe more so since they can see it clearer. But in the end if you say to yourself "I could have done..." you have to know there would have been nothing you could have done. In the end only the addict themself can find a way back.
I agree with you on the whole squash the argument. There were a few moments recently where I can perilously close to permanently damaging my relationship with my sister and realized I myself needed to back off. I took the time to instead strengthen the relationship. So thank you for saying that.
I enjoy your work and just as another person on this ball of rock, I wish you well.