If a charge is disputed to the best of my knowledge it won't (at least after disputing with the credit bureau) affect his credit until a court rules he really owes it. Thats what I would do. Send a formal cease and desist letter and wait for it to drag out. They will settle for less.
This is unclear and confusing. (Also not sure where the court thing comes in.)
You can't send a debt validation letter until there
is a claimed debt, normally once it's been sent to a collections agency (as the providers themselves rarely send debt notices).
And all a debt validation does is exactly that: a demand that the debt be validated. In this case, the debt
is valid, and it will in all likelihood be reported to the CRAs following the validation. Once that has happened, and the credit is negatively affected, it is at THAT point the debt can be "settled." (though the debtor may be given a chance to settle prior to this point)
There's a lot more to it than the above, but that's the general gist.