W211 key replacement?

[2.5Turbo]

My dad is about to buy a 2006 CDI, but the one we're looking at only has one key fob. Since 211's have electronic keys, can the computer be reprogrammed so that the missing key will no longer work, analogous to replacing the tumblers in an older MB? My mom's paranoid about somebody following him home after purchase and stealing it, and we need two keys anyway. The cost of the missing key is being factored into our dealing.

Anyway, about the car...it's pretty sweet. Black with charcoal interior; has navigation and harman/kardon sound. Start it up and you can barely tell it's diesel, plus you don't even have to pre-glow. And it's faster than his Volvo S70 turbo...