You may not need Quad Core now, but if he's building a computer for the future, and he wants 5 clients, AND he can afford it, going quad core is a great idea.

When I was building my computer, I wanted a setup that would safely play the next xpac with 5 clients. The CPU load may not be THAT much more on the xpac, but it will probably be more graphics intensive... just like the transition from Azeroth to Outlands was.

If his budget is $1000, why not get a quad?