Is it permissible to delete pornographic images from a friend's email without his knowledge?

Lewd pictures are a reprehensible act that must be changed. If you can remove these pictures from your friend's site without a greater reprehensible act resulting from it, then you must do so. It is better to advise him so that he removes them himself, especially if you fear that he might restore them or that your action might lead to undesirable consequences. It is not permissible to intrude on others' sites or spy on them.