Rahvin, Be'lal, Semirhage and Graendal were balefired, as was Asmodean, most likely.

Lanfear fell through an interdimensional doorway, Aginor was incinerated to ash by Callandor and Sammael was consumed by Mashadar.

None of them found graves. The way in which Arangar describes the fate of her allies after she's done with them is in no way a unique reference to Graendal's method of dying.

I don't believe it is foreshadowing in any way. If Arangar was involved in her death, even indirectly, then sure. But not in such a random way. Because Arangar was not referring to random misfortunes that befell her allies. She was alluding to the fact that she eliminated them once they were no longer useful to her.

This is not what happened to Graendal.