Although it can depend on exactly what information is sought and requested, and it will depend on all of the facts of the matter, certainly misrepresenting oneself in order to gain unauthorized access to another person's personal financial data would be a crime.
If this is what occurred, I suggest your employer file a complaint with the police.
If, after a police investigation, the police believe that a crime has been committed, and the police can discern from their investigation the identity of the suspected perpetrator, the police could arrest the person and charge the person with a crime.
The prosecutor will review the police investigative evidence and the prosecutor will decide if the evidence can support probable cause that a crime has been committed and that the suspect charged committed the crime. If so, the prosecutor can decide that charges against the suspect will be pursued. If not, the person charged will be released without prosecution and charges will be dropped or dismissed.
Unless private financial data was, in fact, disclosed, I do not see any private cause of action that can be taken by either you or your employer, however. And I do not see that any defamation is involved here, unless you or your employer communicated to another or others that your ex's girlfriend is responsible for committing a crime, without having proof that this communicated statement is true (outside of any police report, where you can report your suspicions). It would be defamation to falsely accuse someone of committing a crime.