High concentrations of Diphenhydramine can potentially cause a false positive for benzodiazepines with laboratories that screen by enzyme immunoassay. Diphenhydramine is found in some sleep aids. Examples are Tylenol PM, Excedrin PM, Unisom and many others.
Oxaprozin a non-steroidal anti-inflammatory drug, can also cause a false-positive for benzodiazepines on urine "drug abuse" panels.
Valerian is reported to cause false positives with some tests
There have been case reports of Sertraline causing false positives.
Depending on the test used changes in pH can also cause false positives (TPDA is notorious for this although she is unsure whether this test is still in use) Here's a link:
Just ask your FOAF's doctor to google "False positive for benzodiazepines". When SWIM did it she got 193,000 citations!
In all disputed cases you should ask for confirmation of test results with Gas Chromatography/Mass Spectroscopy.