N LDAP Server running on OS X Snow Leopard

No, this will not work. You can have multiple versions of OS X on one machine, but you can only install newer versions than the one that came pre-installed. For example, if your machine came with 10.6.4, you can't run 10.6.3 or older.

When Apple releases new hardware, the drivers for that hardware get rolled into a special build of OS X that comes only on that model. They don't get added to OS X in general until the next point release. (That means that drivers for the hypothetical machine I'm talking about wouldn't be added until 10.6.5.)

Apple doesn't release drivers as standalone software, which effectively makes it impossible to install and older version of OS X than what your Mac shipped with. You'll find that if you put the install disc for an older version in the drive, it either won't boot or it will refuse to install.

However, you can go the other direction. If you have an old Mac running 10.5, you can use Disk Utility to make another partition (HFS+ Journaled), then boot the install DVD for 10.6 and install it to that partition. To switch between the two after installing, hold alt when you boot up. (You can also select the default boot partition through the Startup Drive pane in System Preferences within either OS X install.) Boot camp is not required for this. Boot camp is only required for installing non-Apple operating systems like Windows and Linux.

First off he's wrong on all levels.

The Desktop Version is $29, the Server Version is $500.
A standard Mac Mini is $700 base. Mac Mini Server is $1000 base.

Second, the distributions are pretty much identical. The difference is that the Server Version ships the server applications, tools, and processes. Postfix, Dovecot, Jabberd2, OpenLDAP, the entirety of the /Applications/Server directory, and some of the things in /Applications/Utilites like XSan management and the RAID Utility.

If he's looking to do development for iOS, getting the server version makes no sense. If he's looking to do development for Web Services, it's still easier and cheaper to run it on the Desktop version.

If he's looking to run a server, trivially, for mail services, chat and calendaring services, MORE than just web services (web services are trivially easy on the Desktop Version), then he should get a MacMini Server.

Use a Server for a Server for the features it provides, otherwise, use a standard version.

[edit]
We have two versions of XServes, and two Mac Mini Servers (classic style body), I'd be glad to answer any additional questions you or your friend has.