blackmail? by District Attorney

I went to the juvenile court with my daughter two days ago for pre-trial. The District Attorney read the police report to us and then said that my daughter needs to plead guilty to the charge. She knew she has the right to plead not guilty and go to trial. He said and I quote" if you don't plead guilty then I'll add a charge on". She still told him not guilty and he told the judge he is adding a charge on. Is this ethical? legal? She will now have to plead not guilty on both because she isn't guilty.