" A friend insists that he could not be fooled by a digital image that appears to be a photo" - That's probably true, and not for any artistic reason, only simple human psychology. Any image you show to your friend, he is going to suspect it of being digital no matter how realistic it looks; because he knows of your interest and enthusiasm for digital images. To be fair, you would have to collect a group of photographs, and a group of photorealistic digital images, and get him to say which is which.