Defendant's girlfriend should produce the copies of the recording to police, the prosecutor, and the defense attorney.
Possession of the recording should induce the police and prosecution to at least re-evaluate the charges against Defendant, as Defendant would be able to subpoena the gun's owner to testify about the purchase and then use the recording to impeach him if he then denies owning the gun.
It is unclear, though, whether the audio actually has any value because we don't know what charges Defendant is facing. If he's charged with unlawfully owning a gun, the recording would likely be quite helpful; if he's charged with unlawfully transporting a weapon, the recording's value would probably depend on whether the law in question outlaws "knowingly" transporting a weapon or "negligently" transporting a weapon, or transporting a weapon regardless of whether he knew about it.