Without knowing what kind of confidential information was allegedly disclosed to them, don't be too hasty to assume they were actually told the information point-blank. The complainant may have inferred it based on the investigation questions that were asked, for example. Sometimes, in order to investigate the matter it becomes obvious who has likely complained, for example.
HR should describe to you in advance the complaint process -- your rights and responsibilities (especially non-retaliation), the timeline, how you'll hear back, what information you should expect to hear back (outcome substantiated or not), etc. If the matter is substantiated, do not expect to be informed precisely how it was dealt with (e.g., demoted, disciplined, retrained) because of privacy concerns of the other employee(s).