Do you need a special dvd burner to burn dual layer dvds

I just finished spec'ing out a computer build that nearly required a slim optical drive. Frankly, I'm not a fan of slot loaders at all, so there's not one I can particularly recommend.

But mounting a slim drive in a full-height 5.25" bay should be doable. Not pretty, but doable. I don't know of a mounting bracket offhand, which is what you'd really want, but I expect you'll be able to juryrig something if you can't find an existing product.

You'll need a slim SATA 7P-to-std adapter cable (like this one -- this is an example, not a product recommendation) to hook up the final product. The optical drive itself won't come with one (unless you're buying a retail version instead of an OEM drive).

Update:
Thanks to Molly's answer, I found the Digistor company that provides bezel kits (including single-slimline and dual-slimline mounts) and a 5.25" SATA slot-loading drive (available in DVD-RW and Blu-Ray varieties):

It could be an issue with the drive controller it is plugged into, or it could be an issue with Windows. Did your cousin use Windows 7 to burn DL discs? Also, have you tried your burner in your cousin's computer?

Reinstalling Windows may help, but there is no way to know until you try.