There has already been a ton of great advice in this thread, and I believe it should be followed, as it is the key to getting this animal back on track. With that said, I have a few 4-5 year old females that are 500 grams by choice. I feed them every week like everyone else, and they just don't eat very much. Maybe 4 times per year. I have noticed a few that were like this last year decided to turn on this past year, and went from 500 to 1300 grams or slightly higher in one year. Sometimes they just grow like that. I am not stating however, that the animal you just received was treated that way, but just wanted to show that sometime looks can be deceiving.
Good luck with your girl.