If you want to please Yenn you can either use the clothes you got from your visit from the Emperor or buy them again from a crossdressing Elf called Elihad. The items I used were; ElegantCourtier's Doublet, Nilfgaardian Trousers and Elegant Nilfgaardian Shoes. If you're going to go that far don't drink at all and fight the Madman dude in Yenn's honour. Goodluck