You say she is on medications it may be good to check out the dosage and if they need changing.what does the Doctor say ?
See a priest sounds like a bad spirit no joke
u said your daughter has no troubles at school so she has never told her friends in class or a teacher of this friend?
what was the lie told to your daughter
does she say this ghost has an aura or does she see colors around people
as to having sex; what exactly did she tell u since it is not possible to have physical sex with a ghost
do u have other kids
what of other adults in the family; grandma, grandpa, aunt, uncles etc; has she told them about this ghost and has she told them of these changes
what does she tell u as to how she feels about all this and when does she say she communicates with billy
what does her dr say; is he a Psychiatrist and what meds is she on and since when
why did she need a Psych Dr at age 2 and has it been the same person since 2
good luck
Have you researched her medication to see if that can cause delusions?