He is 100% incorrect. XP Home will support a dual-core CPU. Everyone here will back me up on this.
Here's where your friend's confusion comes from... Windows XP Home (and Professional) pre-date dualcore CPUs... so way back in 2001 when XP first came out, Microsoft made it clear that XP Home would NOT run on a multi-CPU system... in other words, if your computer had 2 CPU sockets on the motherboard, you were going to have to purchase XP Professional. Microsoft still sticks by it's mantra of "1 socket = Home, 2 socket = Professional" but your dual-core CPU sits in one cozy, little socket.