Was he given xanax via IV or did he get a prescription for xanax from the ER doctor?
IF it was administered by IV WHILE in the ER, the medical records and discharge summary should show that, and he may be reinstated.
IF he was given xanax while in the ER and was given a prescription and filled it, took it, and did so without the consent of his PM doctor, he will likely still be dismissed.
Most PM offices and contracts explicitly spell out that treatment (controlled substances)while IN the ER are permitted with documentation, BUT if a prescription is given, the patient MUST contact their PM office to get approval BEFORE filling or taking any scheduled meds.