Private investigators delve in to people's lives.  How some do it sometimes does border on legality.  But here you make a connection with the creditor and that may be where you have an up on them.  Creditors are limited to what they can say and do in an attempt to collect a debt under the Fair Debt Collection Practices Act.  I think that if you can show the connection between the investigator and the creditor and call him or her their "agent" in this circumstance then you may have a complaint to be filed against the creditor and the investigator for a violation.  I would call the State Attorney General's Office for starters.  And then I believe the FTC.  Good luck.