Unless her spies reported that Rand has balefired Semirhage, and even then.

If she's not learned about this, then Graendal would never have believed Rand capable of killing her, let alone destroy a whole palace full of innocent people. Whatever elaborate trap she would have tought he was setting for her, this would have been among the last things she would have expected. What Rand did Graendal would have expected of Demandred or any of the others, but not of Rand.

If she's learned about Semirhage and had spies reporting how much he's changed, then she could have been a bit more on her guard. But I doubt that.

But I too think she got fooled and died for it. Most of the Forsaken would have. They expect al'Thor or LTT, they had little idea they were facing a monster closer to Moridin by mid-book.