Blog spammers have sunk to new lows.
The implications of this are disturbing. His blog was hacked (which isn’t unusual and could have been for a thousand reasons like another account on his server being hacked, and old version of phpBB or other software) but instead of doing anything obvious to disturb the content of the site they invisibly modified his posts using CSS-hidden text. He has probably had hundreds of posts modified. I can’t imagine cleaning it up will be pleasant.