PP You say "I dont think there is any difference between the employer working closely with the police and a supermarket down the road
they all have to follow their own rules"
There could be. The police may insist on the check as a condition of the contract with the firm.
Clearly, we don't know enough about this to make a worthwhile judgement. For example, who told him he had failed? Was it the police or the employer? Has he asked for the reason? What was he told? How much of this is in writing - did he do the vetting check in writing or was it verbal? Is there a written decision from the police? Was it the police who decided he failed, or the employer?
I'm not suggesting answers to all this are posted here - just indicating it can be quite complex & he needs to do whatever he can (Including DP Act request if necessary) to get to the bottom of it.