OK, with a little more digging and a few contacts here is the references needed with regard your situation!!
Depends on the state in which that person relocated.
Each state has their own collection laws and they all have to deal with the Federal laws that also deal with that topic.
If there is a judgment or just a debt?
If it just a debt, you can find the US federal law on debt collection practices in Title 15 of the United States Code.
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Debt collection in the USA is big business and is governed by the American Collectors Association with the commission rate charged anything up to 50%
of moneys collected or paid to the creditor and like this country there are agencies who undertake NO COLLECTION NO FEE. What I am saying is that the fees in the USA are higher than in the UK.
Do some of your own groundwork to put your mind at rest/unrest, you decide.
Call or send an email to a debt collector in the US and ask them if they can recover monies for you, from a British citizen, residing in the US - I know what the answer will be
The Knowledge Base of the Credit and Collection Industry
The question of collection through a US court would be undertaken by a Lawyer in the relevant state and as with this country these legal fees are recoverable and added to the debt.
The relevancy I believe, is the amount of the debt, allied with the ease by which a contact is sought, if for example the debt was with a large worldwide bank then they would have people in place already to deal with debtors moving abroad.
DWD