If I am the fey queen, and an insolent mortal sorceress angers me by being insulting or threatening, I will call on the Eldest to lay a curse on her tongue. Until she appeases me, no one will ever believe her words. If she tells the truth, people will assume she's lying. If she lies, they'll believe her words are true. And regardless, they will believe these things even with solid evidence staring them in the face.
I won't tell her about this. She'll know I did something, but I won't tell her what. She'll just have to figure it out.
Her party members will be exempt from this curse (unless you want to go to the trouble of enlisting the other players). So will I, because it wouldn't do for her to be unable to offer apologies. If she does apologize, perhaps I'll petition the Eldest to undo the curse. Or I may demand some service of her in exchange. Who can say? It may depend on what I had for breakfast that morning.