Is there a plugin which can allow a user to email a photo from zenphoto to someone elese?

No, sorry, there is not.

Not a plugin, but I've been using addthis (addthis.com) to provide a button that e-mails links to photos in addition to enabling posting at various social networking sites. Their site generates the code, which I inserted on image.php (in the StopDesign theme folder) where I wanted the button, which in my case was right after printTags at around line 37. You can see the button implemented here or any other image page at vista.howlingpoint.net/z.