also for druids to duel you need
17 charisma
17 wisdom

My buddy and I are going through the game in mulitplayer mode. He created a "vanilla" fighter. He just went to level nine, and the plan was to dual class him into a fighter/thief or fighter/cleric. Unfortunately, his dual class button is blacked out, and for the life of me, I can't figure it out. This is my second time going through the game, and his first, so I'm supposed to be the "expert." Please help so I don't look like an arse. Thanks.

Firstly, the Character needs to be Human. Secondly, you have to have 15 in the main ability for the current class (Str for a fighter, Dex for a thief or Wis for a Cleric) furthermore you need to have 17 in the main ability for the class you want to dual to. I hope this solves it, otherwise just search for Dual-Classing in the search part of this board.