You do not say how your friend was convicted, if it was by plea or if he had a trial. But in either event, please understand, that if he did not accept the plea offer the prosecutor can revoke it at any time, he does not have to honor it later. In fact the prosecutor is under no obligation to offer any plea agreement or sentence cap at all. Thus if your friend did not accept the offer of 21 months and put it on the record with the court, it does not matter if the prosecutor admits making it or not, he does not have to honor it.
From the form of your question, it does appear your friend's attorney did advise him of each potential plea offer that was made, as well as the potential penalty if he did not accept them, and rather than accept any of them when made, your friend elected to try and negotiate a better position, which the prosecutor was unwilling to do.
Answered on Apr 15th, 2013 at 8:39 AM