The "D" nav discs are no longer offered by the manufacturer, NavTeq; one is told to contact the dealer to buy them. That tends to indicate they are being or have been discontinued, so your information may be correct.
But before spending $300, wescoass really needs to determine whether he has "C" or "D" nav. It is very simple to do (it may require that he already have at least one disc).
If he has no disc at all, then he can check the date if manufacture of his car on the plate inside the driver's door. The changeover month from "C" to "D" was during November, 1999. Unfortunately I do not have the exact date. Cars made during the first half of Nov. 1999 or earlier are likely "C" nav. My '00 was manufactured in the last week of Nov '99 and had "D" nav. However, because the nav system may have been replaced due to malfunction in intervening years, a car made before the changeover could potentially have the newer system.
If he has "C" nav, the last of those sets was released in early '04.