You just need to corrupt one to give you whatever pass you need.
That's how the "Adolf Hitler" pass was generated a few months ago.
If he didn't have any symptoms, and was doing PCR test regularly just to be sure, he had to reason not to go to the gathering.
And given they were ready at 8pm it’s quite possible that the guy that makes Djokovics mail account was long gone. It may even have landed in the Spam folder if he is using Gmail (happened to me once).
It’s up to the court to decide what happened. I guess that Serbian courts can get the details about the results delivery. All it has to do is ask email providers to check logs, both on the sending and receiving side.
As always, it’s the one making accusation that needs to provide a proof.
On the other side, he can claim before Serbian court that all is five and the test was fake, but he can’t be processed about that in Serbia, since he neither faked nor used it there. And in Australia he can claim test is valid, and Australia would need to prove it’s fake if the want to cancel his visa. Which they cannot prove that easily, since Serbian authorities are not obliged to jump on immediately on the Australian request.
Yes, it would be a loophole and playing two legal systems, but that’s what companies do all the time when it comes to taxes.