Send a letter return reciept requested. Saying you have no knowledge of the debt and that it is past the statute of limitation (check that, but after 9 years. Cite the Fair Debt Collection Practices Act and ask that they stop contacting you in anyway. Then check you credit report every month for the next 6 months to make sure they try nothing funny.
I have a collection agency and we abide by the FDCPA to the letter.
If a debtor or alleged debtor for that matter writes me a letter like that, I send the file back to the client and tell them to get another attorney to sue or write it off.
The potential liability on my end is too high to screw around.