Are you saying that his Macbook has two drives inside it at all times, or that he's swapping drives in and out of it?
I think, if I understand you correctly, you're talking about one of two situations, and actually, I'm not clear that the swapping is really relevant to either of them:
1) He wants to have one OS on each disk. This is completely legal for OS X; for Windows, the Update disc requires you to have a license for a previous version of Windows... as long as you've got that license somewhere, that other copy of Windows is not in use, and you've gotten it to install, my understanding is you're legal there. But the Update does require you to own an eligible previous Windows, AFAIK.
2) He wants to have both OSes on both disks. The caveat applies. Above and beyond that, it's a bit of a grey area, as I understand it. Many of us make clone backups routinely, and I consider these fully legal, as they do not circumvent any kind of copy protection and are used only for archival / backup purposes. So I have two hard drives on my iMac and a total of three copies of my iMac's Tiger environment installed across them, but they are never used simultaneously. His situation seems analogous as long as the drive that is not in the Macbook is not also actively being used by another computer.
So basically, he can swap if he wants to, but in order to be using a Windows update disc fully legally, it needs to be tied to an eligible upgrade product, AFAIK.
