My players guessed a plot twist.

So I am running a revamped version of LMoP and basically to provide some context, the party was asked by a common friend of theirs, Gundren Rockseeker, to deliver some supplies to a nearby town and then way for him to tell them what to do after that. He also asked them to retrieve from a crypt a mysterious box that, after they opened it, they found out contains a small stone with a rune engraved onto it. The following day a party member had a vision of a hooded figure with long white hair and doppelganger subordinates stormed the crypt only to find out that the box was missing and acknowledging the existance of Gundren and what he wants to do. While traveling ot the town they found out that Gundren has been kidnapped and taken to the hideout of a goblin tribe who work for a person called the Black Spider. They want to know the location of an ancient mine in the area that Gundren rediscovered and a magical area within that allows the creation of magic items. Then I introduced them to a NPC called Nezzar, a human woman with short black hair and a scar who asked them to trade the box with something they wanted, and, after saving their lives multiple times to gain their trust, the players agreed to give it to her if she found their friend.

Now in reality, Nezzar is the figure they saw in the vision and she is the Black Spider, a drow mage that has cast disguise self or some other spell to change her form. She wants the stone because it can be used to banish the guardina of the magical place in the mine. The players were going to give her the box, whose contents replaced with some bell bearings and a minor illusion spell, for Gundren and, as she left, it would be revealed that this gundren is actually a doppelganger and that Nezzar is a drow and the black spider and she will fight them, very disappointed because they tried to trick her and now she has to kill them.

Some time ago one of my players sent me a messege telling me how she and one other player talked about it and thought that this might happen using evidence that even I didn't intend to be hints, like her refusing to take them to gundren because they would be a "liability" (she wanted to use dimension door and didn't want the players to see her). I don't really know what to do now and despite the fact that IF they decide to tell the other players their thoughts they might try to find a way to trap her but I still fell frustrated and like this is unfair. Do you have any advice on what to do?