You can detect it in hair, so, we can detect it back as long as there is hair growth. That's likely six months, which is long enough to validate the claim.Only is said test is done within a whatever perioud crack stays in your system. Otherwise it only proves he hasn't had any crack in a given timeframe.
And honestly, if he hasn't smoked any crack in the last three months, then he isn't an addict, and even if he did sometime before that, who cares.