Well, to use a legal analogy, goes to character...
If someone you depose has their veracity proven to be unreliable, the conclusion might be nothing they say can be trusted. Might they still tell the truth - well, yes. But is the entire statement after the lie in question. Yes
So goes addictions. Could a person who was an addict not get addicted to another substance. Perhaps. Might they, well yes.
The math of addiction rarely adds up to empirical conclusive data. Hope for the best for your friend I suppose.
peace