So far as I can tell, the neighbour isn’t a party to the proceedings. The allegation of defamation is against CASA, as a consequence of the disclosure, of the complaint, to the pilot’s employer.
All CASA has to do is prove is that its disclosure to the employer is subject to qualified privilege. Unless the pilot can show that CASA knew the complaint was false but nonetheless disclosed it to the employer for malicious purposes, my money is on qualified privilege applying. When in doubt, “safety” must prevail over mere bagatelles like reputation, career and natural justice, don’t you know...
Qualified privilege applies to some disclosures precisely because they may turn out to be untrue and defamatory.
You should google the term “qualified privilege”. The word “qualified” in this context has nothing to do with the qualifications of the person making the defamatory statement. It’s about the restricted circumstances in which the privilege applies.