Is it a dick move to blindside the party with an illusionist they have no idea is coming?

To be clear - this is about the social/RP side of things; if they were dungeon crawling I'd say pulling illusions on them is absolutely fair game.

The PCs are about to go rescue their princess friend, who surrendered herself to the BBEG and has been kept hostage in a palatial gilded cage. Before she gave herself up, she gave the PCs a key MacGuffin, a special gemstone necklace, sacred to the royal family - unbeknownst to the party, it's the firing key for the superweapon their nation was built around. The BBEG wants his hands on it for his master plan.

The party has been communicating with her using Sending, so they know some things:

• Her warden is an elf lady and uses some kind of magic, since she's been using spells like Zone of Truth to interrogate the princess as to the whereabouts of the MacGuffin
• The warden was a star pupil at the local magic academy before getting expelled because she was a trickster known for cruel, malicious pranks (due to timing, they can't go back and ask more about her)

I'm thinking that when the PCs come raiding the palace, the warden casts a sleep spell on the princess, stuffs her in a closet, and uses an illusion to make herself look/sound like the princess, so she can get them to give her the MacGuffin, which she then delivers to her boss.

Have I adequately foreshadowed this? Or is it a dick move to just go "ha, it was an illusion, you gave the BBEG what he's looking for" out of nowhere?