It depends what was viewed. Obviously the client must open the message, but if you view the message source you can see the URLs for any images encoded in the links.
In my case I wrote a little Outlook macro to save suspect messages as a text file for further review. Outlook has to read the message into memory so the macro can grab it, but Outlook never takes action on any of the links.
My guess is that "Luke" now has a temp file on his computer that he doesn't want there, and that he doesn't know is there. If he's lucky, it's in a temp folder, it'll get "deleted" into the Trash, he'll empty the trash, and overwrite the data sectors before the Feds decide to search his computer someday, or he sells the computer without wiping the drive and some -other- poor schmuck has it.