canada
Tribunals vary greatly on the degree of disclosure they expect between parties generally or in any particular case. The degree of disclosure may be dictated by statute or be driven by the common-law requirements of procedural fairness. If the latter, this will depend on factors like the right or interest affected, the nature of the tribunal decision being made, the importance of the decision to the person demanding disclosure, whether the Tribunal itself has provided rules concerning disclosure, whether the party has availed themselves of Tribunal processes to compel disclosure, etc.
Therefore, how a Tribunal would view the circumstance you describe is unknowable.
However, in the scenario you have crafted, I question the relevance of the material from the investigation. If employee X's allegation is that they were fired for whistleblowing, what is relevant is the concern employee X raised to the employer and the employer's resulting actions against that employee X. Whether the employer also happened to investigate those concerns does not seem relevant to any whistleblowing protection regime I am aware of.
Whistleblowing is defined based on what is communicated from the employee to the employer. See e.g. the Public Servants Disclosure Protection Act, defining protected disclosures in relation solely to what was communicated to the employer:
any information that the public servant believes could show that a wrongdoing has been committed or is about to be committed, or that could show that the public servant has been asked to commit a wrongdoing
The Act then also protects against reprisals for such protected disclosures.
None of this depends on whether an investigation was conducted nor the outcomes of any such investigation.