except for some antibiotics, and certain diseases related to the kidneys or liver, iirc (don't quote me).
never say never, especially in biology.
When you're on probation, they do a preliminary test. If it test positive, which is possible in the cases you're giving, it's given a second test. And in the confirmation test, there is simply no way. It will detect ONLY specific metabolites.
I used to work for Qwest Diagnostics. Believe me, you can dupe a test, but you can't dupe a positive when they do a secondary test.
If they did not test the sample twice (which has it's ups as well, as most drug metabolites are undetectable after a certain amount of days) the lawyer could have a field day and demand some other type of test if the parole/probation officer tried to violate the person in question.
However, that could be bad, because the lawyer could demand some other form of test. Now, if the parole/probation officer is not that intelligent or on a budget, they could do a saliva or sweat test, which would be clean by now. Or they could do a hair test, in which case the person is definitely going to be violated if they had used within X amount of days.
But the probation/parole officer and judge also have a lot of discretion in many cases and aren't necessarily obliged to correct anything.
As well, if it's a first violation, generally the penalty will PROBABLY be minimal.. The justice system just doesn't have enough money to lock everyone up for a minor violation.
Either way, I'm sill betting that the guy is a liar.
And he really needs to consult a lawyer rather than having his girlfriend ask us on BL like it's going to do anything. What, is the person in question going to show their parole/probation officer "oh, look at these post on BlueLight.org, it could be this, this or that!"