If a prosecutor strikes a deal with the defense, and the plea agreement is filed with the court, the prosecutor is bound to it. It depends what the deal is, however. For example, if the plea agreement requires the defendant to do something (like give substantial assistance to the cops, do work as an informant) and they don't, then the prosecution can ask for the court to depart from the agreement. But otherwise, if the agreement is filed with the judge and the prosecutor changes his mind, generally the agreement still remains.
There aren't enough facts provided here, but it sounds like a plea agreement was entered and the sentencing was set off. The reasons why sentencing was set off would need to be known.
This person should discuss the situation with their attorney to figure out why the prosecutor is not wanting the court to honor the plea agreement.