Such exemption may be valid if a third-party asks for a copy of a complaint; that'd constitute an unwarranted invasion in the privacy of your father. But not to an appllication by the applicant (father) seeking a copy of complaint against the applicant (father) himself. At the most, secrecy of the complainant may have to be protected in terms of Section 10 of the Act, but denial of information in its entirety, in my opinion, is not justifiable.
In A. L. Agrawal v. Delhi University, the applicant had sought a copy of complaint purportedly for s*xual harassment made against the applicant by students underaking research work under the applicant's guidance. The PIO denied a copy of the complaint to the applicant. Referreing to the Sexual Harassment of Women at Workplace (Prevention, Prohibition and Redressal) Act, 2013, the service rules applicable to the applicant and principles of natural justice, the Central Information Commission had held that there was no prohibition on sharing of the copy of complaint with the complained officers and that principles of natural justice demanded that the copy of the complaint must be given to the accused officer so that he would get ample opportunity to defend the charge made against him.
In Ayyappan v. Bairavaswamy, the Central Information Commission turned down the defence of Section 8(1)(g) of the RTI Act (endangering life or physical safety of any person) and directed the PIO to supply the police complaint lodged by the applicant's wife against the applicant and his parents regarding dowry harassment, safety and security.
In R. K. Gaur v. D.G. Vigilance, the Cenral Information Commission directed PIO to provide information on the nature of the complaint filed against the applicant without revealing the names and address of the complainant, by applying Section 10 of the RTI Act.
I'd suggest that you approach the First Appellate Authority and challenge the decision of the PIO on the grounds that:
A. It is the applicant's personal information and thus not exempted from disclosure under Section 8(1)(j) of the RTI Act to the applicant qua the applicant himself;
B. The authority has no fiduciary relationship with the complainant; if anything, there is a fiduciary relationship between the public authority and the applicant;
C. It not as if the disclosure of information will reveal any confidential information, the applicant already being aware of the complaint made by the complainant (as evidenced by the specific RTI application which was denied on the ground of confidentiality, not on non-existence);
D. That the principles of natural justice require that the applicant be aware of the allegations raised against him;
E. Strictly without prejudice and in strictly in the alternative, if it is considered that the complaint may have any information, the disclosure of which may impede the investigation, if any, or reveal any complainant/witness, the same can be severed before supplying the information, but subject to strictly establishing that such severance is absolutely necessary.
Even otherwise, in the event that the authorities intend to take any action against your father or investigate on the basis of allegations made in the said letter/complaint, then you'r father has a right to have a copy of the letter and to receive an opportunity of being heard to defend himself against the allegations contained in such letter. You have already brought it on record that you sought a copy of the said allegations and were denied; retain a copy of this communication and PIO response and don't worry; if the authorities take any action against your father without hearing his version, you'd have a good ground to challenge any such finding/action and authorities might find it hard to sustain such an action. Heck, it may even be cruelty to make false imputations about the father of a spouse!