Actually that's true, in retrospect I don't know if he's ever stated to be a darkfriend, Graendal just calls him one of her "tools." It's likely he was just a fanatic that she had manipulated, possibly under compulsion. But it does seem like it wouldn't take much to pull his strings.
It's in ToM, when he attacks Perrin during the fight against the Trollocs, and Bornhald ends up killing him to save Perrin's life. I think after that Graendal either thinks to herself or mentions to someone else (Moridin or Shaidar Haran, possibly) that he was one of her agents.
I believe the letter she sent to Rand led him to returning Ituralde's king, who had been taken by the Aes Sedai for his protection.
This thread has some discussion on what Alanna's letter may have contained -
http://www.tarvalon.net/phpbbforum/view ... =36&t=6835
I'm not sure if anyone's...