It's ironic foreshadowing, because the author made Aran'gar's thought in KoD - which I agree was cynical, because actually she killed her allies - come literally true: her new ally died as all the others, misfortune actually played a part in this, Graendal wasn't useful anymore in Arad Doman & in the story, she didn't even have a grave because of her balefire death, and the other characters & we readers won't hear about her affiliations to Aran'gar from Graendal anymore, since she took this knowledge with her, when she died.

It's often the case that foreshadowings basically announce the core of what will happen (though I think Aran'gar's quote is very obvious in its irony): when e.g. Rand tells Min in LoC: "Trust me, Min. I won't hurt you. I will cut off my arm before I hurt you", it might foreshadow that Rand will lose his hand and that he will hurt Min, though the circumstances are not exactly like the foreshadowing.

I'm pretty sure that Aran'gar might muse on the irony that this time she didn't even have to kill her new ally, because it truly was this time in a way misfortune that killed Graendal. Again, Aran'gar outlived her ally.