f1.8 isn't possible with his kit and you usually want to drop a couple of stops to at least reach the sharp area of the lens. F11 will make sure everything is in focus too, not much point getting a bit of one person in focus really is there? and upping the ISO isn't a bad idea but isn't 1600 a bit noisy for portraits? Personally I'd use a flash to fill in and set it down on power or use a diffuser so its doesn't cast horrid shadows.
But that's just me. I'm no expert!![]()
On a 40D ISO1600 will be great. But then I shoot B&W so its not an issue.
Find out if the image looks ok or not and make your own decision. Its digital. Take a risk, delete if need be.