Or would you rather go for ?
He met another ghost called Camilla.
Though not a teacher, my preference would be your initial suggestion of a "female ghost" or a "ghost of a female/lady/girl".
Building on Anglika's suggestion, might I suggest: "He met a ghost, called Camilla". This avoids the presumption of at least 2 Camilla ghosts.
I'm not a professional teacher, but in the context of a story about ghosts, I would be comfortable with "she-ghost". It follows the pattern of she-devil. I have to stress that this is an aesthetic judgment on the merits of the term in the context of a fantasy story. If I were trying to be gender neutral or writing about the ghost in a matter-of-fact style, as if she were a real female person, then I would simply say, "a female ghost" or "a ghost. Her name was Camilla."