This is as though Apple were hotlinking his images -- they are being consistently used in a setting besides that which was originally intended, without permission (or even attribution).
I don't see why putting your url on your own content is 'playing dirty', but displaying someone's stuff without telling them you're doing it or even attributing it sure is.
He could always play dirty and just print the website on the bottom of the image but I don't see how this or the c&d will help his site.