If the photographer was not willing, force was involved.

If the photographer willingly gave it over, he or she is either too trusting, or rather naive. They have no way of knowing what you or I would do with it, or if we're tech-savvy enough to delete the one image without messing something up.

If the photographer was willing to let you delete the image, they'd probably be willing to do it themselves. More willing than they would to hand over a god-knows-how-expensive bit of tech to a stranger.