You should call Dunn and Bradstreet, they have a collection department, if they can't collect they will put it on your clients credit report.

Normally the nationwide companies work directly for the lender, and since I use to work directly for banks years back, I happen to know for a fact that they check your companies credit before bringing you on as a vendor. And this is one of the places that Banks check, D and B.

You might even send them an email telling them you will be doing this. It might scare them enough to pay up.