Differences between Snow Leopard Server and Desktop?
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.