Jordan was telling us that Aran'gar was deceitful and might try to kill Graendal once she's got from her what she wanted. Graendal didn't die at all like Aran'gar's former allies have died (by "accidents" caused by Balthamel), so I can't see how that remark could have been foreshadowing.

You really have to make that quote dance on the head of a pin to extract anything foreshadowing Graendal's death in it....

The irony of that remark is that so far it's Graendal's "allies" who tended to drop like flies: Rahvin and Lanfear have, and later Sammael, and she apparently had let Asmodean know where she hid too. Yet Aran'gar also showed herself deceived by Graendal's fake superficiality, which didn't bode well for Aran'gar's long term survival, especially when Moridin had reflected earlier that those who made the mistake of underestimating Graendal paid with their life. But apparently, it will not follow that course.

There's really no foreshadowing there, unless we miss something big about Aran'gar playing a direct role in Graendal's demise, but there's really no sign of that in TGS (sometimes a cat is just a cat. Graendal was incited by promises of rewards to remain in Arad Doman when Rand showed up, something that earlier on she didn't plan on doing. That wasn't wise to change her sensible plan to flee when he set foot in AD - her greed for power and rewards was her undoing. Rand outwitted her and killed her. There's most likely little more to it than that - we have to remember that on the timeline this took place at nearly 2/3 of the final book, it's the time for villains to start falling for good, not for ultra convoluted stuff Jordan would not have had the space or opportunity to resolve in the last third of the book. )

Jordan was not hinting at her imminent death in KOD, he was setting things up about Aran'gar. We don't know if anything will come out of it or if this was a red herring. In the prologue of TGS, Graendal reflected on this new alliance with Aran'gar, so it went forward and it sounds obvious that we'll learn at least a bit more about it later. We are going back on the timeline in ToM, so a scene in which Graendal appears, set before her death, is not out of the question. Or we could get a POV from Aran'gar (before or after Rand killed Graendal), developping her toughts on the alliance, and her intentions/goals now that Romanda has forced her to flee. Before that, Aran'gar had plans to use Graendal's network of DFs, in ways that Graendal wouldn't approve of. Well, with Graendal's death she doesn't need to worry about doing that without getting caught by Graendal, she can simply take over the network. That's if Aran'gar is still alive. There's at least three story lines left in ToM in which she could technically get herself killed before Rand killed Graendal!

The question is where did Aran'gar go after Romanda exposed her? She had those plans involving Graendal's network and, to paraphrase her, "she'd disappoint Moriding about not killing Rand". She may have visited Graendal, but it's unlikely she set up shop in Arad Doman (Sammael said Graendal didn't trust DF and we know she had none or few around her, so obviously the "huge network" Aran'gar spoke of is active outside Arad Doman, not inside it...). Aran,Gar had also recently found out that Mesaana was in the Tower. Aran'gar possibly knew that Osan'gar once infiltrated the Asha'man like she was ordered to infiltrate the Rebel Aes Sedai, and she had an alliance of a kind with him. If Aran'gar met with Graendal, it seems likely Graendal would have tried to convince Aran'gar to focus on Demandred/finding Demandred so that together they could bring him down, especially now that Demandred is at least temporarily in a position of weakness, with Moridin displeased with him and Mesaana, the "trio" is down to a duo of Demandred and the weakest link of their alliance...

The Black Tower is one place Aran'gar could likely be tempted to infiltrate, especially if she knows that Taim is a darkfriend. (she could guess that Demandred is involved there one way or another. If he speaks of an army and getting ready for war, that'd be the obvious place to guess he's at, for Aran'gar.. It was a little contrived in TGS that Graendal didn't even considered the possibility Demandred was involved with the BT). That's also her best hope to rebuild herself a power base fast, and from her POV it would place her at the best place to make the alliance between male and female channelers fail. It's that emergering alliance that has caused all her problems and ruined all her plans, so revenge would be sweet to her, and she's failed to ruin it from the Rebel Aes Sedai side.... An inverted mask of mirror and she is a man, channelling saidin, posing as an Asha'man to control Taim. At the Black Tower, she could plot against the Tower, Mesaana and against Rand all at the same time, and perhaps against Demandred as a bonus. It's clear Taim answers either to Demandred or to Moridin. Aran'gar could even have been the one who made Egwene's envoys seemingly disappear (she knows fairly well several of these women, and her pet Delana knows them even better). And by taking control of Graendal's network, she could cause damage, especially if she disguised herself as Graendal to give orders Moridin wouldn't approve of...

If you want foreshadowing of Graendal's demise, it's in the Sammael and Graendal scenes. She tried to play her fancy mindgames with Sammael, and he defeated her for a time by refusing to play, taking her by surprise and bullying her. And she agreed to go along because she couldn't take the chance that he was telling the truth about being promised Naeblis, and if he was, she needed to stay on his good side for as long as she couldn't bring him down. That's pretty much what happened to her with Moridin, while Rand brought her down pretty much like Sammael has done.