My son is on probation and he is tested on Monday, Wednesday, & Fridays. If he were to drink on Friday or Saturday would this be detected with his urine test

If Saturday is the last day that he drank, he should urinate it all out by Monday with the help of drinking water.

Of course if a friend gets mad and reports him to the PO, So why should you be accepting the fact he is not following his Probation rules.