End of Bonehunters question (spoilers for end of the book)

At the end of bonehunters when Grub and Lostara watch Eres’al rise from T’amber, Grub tells Lostara to keep this secret from Tavore.

But if Tavore didn’t know she was used by Eres’al, why would she think that T’Amber was capable of fighting the way she did?