The only parallel between Aran'gar's statement and Graendal's death was that she died. Aran'gar is clearly stating that she allied with people in the War of Power and then killed them later when she'd used them. That has absolutely nothing to do with Graendal being balefired by her arch-enemy, the Dragon Reborn. Saying that Aran'gar is foreshadowing Graendal's death is as true as saying that Sammael foreshadowed Lanfear's death when he said he'd kill her.