Drug tests nowadays are pretty specific. He should ask for a second test to confirm, as one of several things may have occurred:
He may have broken his nose, or recently had surgery or a procedure involving the nasopharynx... in which case, he may have actually been given cocaine and not realized it.
He may be on a pain med that contains cocaine (i.e., Brompton's Solution, a mixture of morphine and cocaine) and not realize it contains cocaine.
He may drink tea containing coca leaves, and have a non-false positive result.
The lab may have messed up the test... It does happen; they are human.
(It might be helpful to have a complete list of meds... including herbals)