Here's what I'd do if I were in your position. Get a snailmail address next time they call. Write them and tell them that they're in violation of 15 USC §§ 1692b(2), 1692c(b) [presuming that you are not the debtor/consumer, which you are not]; that you do not wish to receive any further communications from them; and that if they communicate further with you under the assumption that you are indeed the debtor/consumer (except to the limited extent provided), they would be in violation of 15 USC § 1692c(c). Send it certified mail, return receipt requested.

Also write separately to American Express at the contact address you have on your card. Tell them that their debt collector is in violation of the first two statutes, that AmEx has provided inaccurate/outdated information to their debt collector that they've failed to cross-check against their current accounts, and that you will cancel your account if you don't receive confirmation within 30 days that the matter has been resolved.

Caveat as always, this is not legal advice, I haven't practiced consumer law in over 15 years, and I'm currently on inactive/retired status.