Here we go, this is all from
http://www.ftc.gov/os/statutes/fdcpa/fdcpact.htm which is the Fair Debt Collections Practices act. Basically, your rights if someone is trying to collect from you. Seeing as you are in fact a third party, article B will be especially pertinent to you. Article C is pertinent to the actual debtor.....but note that the debtor can tell them to stop, and they must comply.
(b) COMMUNICATION WITH THIRD PARTIES. Except as provided in section 804, without the prior consent of the consumer given directly to the debt collector, or the express permission of a court of competent jurisdiction, or as reasonably necessary to effectuate a postjudgment judicial remedy, a debt collector may not communicate, in connection with the collection of any debt, with any person other than a consumer, his attorney, a consumer reporting agency if otherwise permitted by law, the creditor, the attorney of the creditor, or the attorney of the debt collector.
(c) CEASING COMMUNICATION. If a consumer notifies a debt collector in writing that the consumer refuses to pay a debt or that the consumer wishes the debt collector to cease further communication with the consumer, the debt collector shall not communicate further with the consumer with respect to such debt, except --
(1) to advise the consumer that the debt collector's further efforts are being terminated;
(2) to notify the consumer that the debt collector or creditor may invoke specified remedies which are ordinarily invoked by such debt collector or creditor; or
(3) where applicable, to notify the consumer that the debt collector or creditor intends to invoke a specified remedy.
If such notice from the consumer is made by mail, notification shall be complete upon receipt.