The French Supreme Court answered this question in a decision dated March 17, 2021 (Nr. 18-25597).
In this case, an employee was dismissed for gross misconduct on the grounds that an investigation led by an external company (which was appointed with the employee representatives’ consent), had revealed that she had uttered racial and discriminatory insults and caused serious disruption to the company’s organization.
The employee considered that the investigation report could not be used to substantiate the ground for dismissal since she had not been previously informed about the audit nor heard by the auditors. The Court of Appeal heeded this argument and therefore held that the employee’s dismissal was unfair.
The employing company filed an appeal before the French Supreme Court. The Supreme Court overruled the Court of appeal’s decision, considering that the investigation report can be used as evidence in a litigation since no information concerning the employee had been collected by a device which she was unaware of (which would have been in breach of article L.1222-4 of the French Labor Code);
Regardless of the fact that the employee had not been previously informed of the investigation nor heard by the external auditors, the investigation report was then lawfully used to ground the dismissal and evidence the employee’s misconduct.
The decision can be read here (in French though):
https://www.courdecassation.fr/jurisprudence_2/chambre_sociale_576/339_17_46696.html