Shortest fight ever. Voldemort is incredibly effective...in the Harry Potter universe. Drop him in a D&D campaign, and he's basically a lich with delusions of grandeur. He wouldn't last a week in a high fantasy setting. Gandalf/Elminster/whoever would send a group of mid/high level adventurers to his place and they'd have him dead in a few hours.

This is not a knock on the Harry Potter universe at all, mind you. The value of a villain has less to do with how powerful they are, but how well they fit into the story. But Sauron is on a different level of power than Voldemort, plain and simple. That said, I wouldn't bother arguing this with your friend. People take this kind of crap personally, even when they're hopelessly wrong. I once saw some kids on Gaia arguing that Sephiroth could beat Galactus. I had only brought the topic up as a joke, and bam! There go a bunch of Sephiroth fanboys/girls with no earthly idea who Galactus is arguing that he could be beateh by a guy less powerful than the cosmic heroes that Galactus creates.