I agree that Grafdakka probs shouldn't know about the item that could control the Laveithan, though since he knows that the laveithan exists, he does know that someone would want to control it. And who better to try and control it than Victoire, who's demand for control is so great she nearly left the previous battle when her carefully laid trap failed to work as intended? How she intends to control the Laveithan, Graf doesn't know, but better to kill and loot her body regardless.