What finally happens is, she's caught in a gangbang with Britannian officers. They arrange for her to take the fall, knowing that her whorish behavior makes her an easy scapegoat for their own transgressions.
Schneizel does what he can for her, insuring that she is honorably discharged.
THEN Zero approaches her, with the promise of unfettered indulgence of her nymphomania in exchange for her technical assistance with the Black Knights.
Having not had sex in a month by this point, Cecile desperately agrees, and spends the rest of her days fucking six men a day while assisting Zero in his war on Britannia.