Figure out a dessert she doesn't like, and a dessert she does.
Invite close friends over for dinner. Have them bring the dessert she doesn't like. Better if they make it from scratch. Even better if it can be an old family recipe or someone who normally doesn't cook made extra effort to prepare or ingerdients had to be bought from the other side of the city or something. Ask them to talk about (more than once) about how they were going to bring dessert she likes but remembered that she is very particular to dessert she doesn't like. Maybe even piece it into a real memory or two. Like "I remember at the barbeque last fall you were talking about how much you like (dessert) before you told me about (true story)."
You can really play it up while giving friends - who may also want revenge - a chance to play too. After dinner go to the kitchen to serve up dessert "and since you like it so much I'm getting you an extra helping". Friends are excited to see her try their homemade creation.
She's freaking out that she has to eat a huge helping of dessert she doesn't like but doesn't want to disappoint a friend. She's wondering how all these people have specific memories of her saying she likes said dessert. She thinks she's going crazy.
Then you bring out the dessert she actually likes, and everyone has a good laugh.