If the test was done by a SAMSHA certified lab, the test will include an immunoassay screening, and a GC/MS conformation test of the screening result. If he tested positive on the GC/MS after a screening, then he used cocaine. The instruments used for this are highly specific to what they detect, maybe 10-15 years ago he would have tested false positive for such things, but with the high accuracy of testing methods, and the methods used to do the testing, NOTHING will cause a false positive for the cocaine metabolite benzoylecgonine, being as it is only produced when you ingest cocaine. This is the exact reason why they have a conformation test, and the results are legally defensible in court. To be quite honest, if you think he didnt use cocaine, a hair follicle test will prove or disprove the urine results being as it is detected in hair for 90 days after use. The results of a laboratory test are 99.9% accurate, that .1% is due to human error on the test. The machines do not lie. Only the person submitting to the test does.
Certain substances & conditions can cause a false positive for cocaine: Amoxicillin, tonic water, some topical anesthetics, diabetes, kidney or liver disease/infection.