Keira has a plan to sell her secrets to King Radovid, in return for the ruler sparing her life (as he is currently exterminating all the mages of the lands). Agree to her plan, and leave her to be on her way. You don’t hear from her again until Act III: Final Preparations, where you learn she has been burned at the stake at Hierarch Square in Novigrad. During Main Quest: The Great Escape, Triss asks for your help in removing her corpse.
The Witcher 3: Wild Hunt The Complete Edition eGuide Quest Walkthrough: Act I (Velen) Secondary Quest: For the Advancement of Learning