Is Sleeping on Duty a Dismissable Offence?

An employee told his supervisor that he felt unwell and asked for a pain tablet. In response she gave him a tablet which he ingested. Shortly thereafter he went out to buy an antacid remedy but only got as far as the parking lot before passing out in his car. The supervisor went to the parking lot and found him in that state. He was admitted to hospital the next day and his blood test result showed that he had a strong soporific in his system. After his return to work the employee was dismissed at a hearing for the described incident when he was unconscious in his car. At the CCMA the supervisor testified that she had only given him an ordinary pain tablet and that the employee must have obtained and taken the soporific himself. The employee said that the only drug he had taken that day was the one his supervisor had given him and added that it looked very different to the sample of the painkiller exhibited at the arbitration hearing. A few minutes after taking that tablet he felt drowsy and passed out in his car. No evidence was led as to when the soporific entered the employee’s system, and no other evidence was presented. Does the arbitrator have enough evidence to prove that the dismissal was substantively fair?