Answer: Darren is a knight.
Darren asks the waiter "Are you a knight or a knave". If the waiter is a knight he will say so, but if he is a knave he will lie and still say he is a knight. Therefore the waiter will always reply that he is a knight. As Darren correctly tells Dave that the waiter said he was a knight, we know that Darren is a knight (but we know nothing of the waiter).