So is your friend planning on being a healer build for his Templar? I ask because although a lot of people think of Templars as healer, any class can actually do any of the 3 roles, so if he's going to be tanking or DPSing it changes what would be a good build to play alongside him.

Having said that, I find that a DK is easier to build towards survivability, while a Nightblade is easier to build towards DPS. Either class can do either depending on how you build them, but especially early on in the game when you don't yet have access to a lot of the skills you may want to use later, DKs tend to be more tanky and do less damage, while Nightblades are the opposite.

If your friend is going to be healing, I'd suggest going Nightblade. He can help keep you alive while you're refining your build, and while you're on the way to a solid tank build you can be putting out some decent DPS. But it's really just a matter of opinion.