"Change your people, your places and your things" is something I have heard as a recommended best practice for overcoming addiction. I suspect there is some element of psychology involved but probably also physical effects.
It's more or a less a recipe I followed as part of my efforts to address a completely different health issue having nothing to do with addiction. I think we are in the infancy of our understanding of how germs, chemicals, etc. proliferate in our lives and shape them.
And, yes, hopping a flight back to the US and making a clean break with Vietnam -- both the place and the war -- likely helped them make a clean break from their addiction in this case.