First Offender can only be used in a GA court - not a military court. Also, it has to be asked for before an adjudication of guilt. Finally, it cannot be used on a rape charge.
So, for many reasons, no the First Offender Act has no application in this situation.
It sounds like the case was dealt with by the Navy and not a local Superior Court. The Navy and other military branches do not offer first offender treatment and, as Mr. Rhoades stated, first offender is not available for a rape charge. You may consider contacting an attorney for your boyfriend to explore appeal option and other post-conviction relief.
The military courts are different from the state courts and they are even different from the civilian federal courts. Interestingly, a case prosecuted in the military or the federal courts for that matter, can also be prosecuted in the state courts in certain instances. Fortunately, Georgia has a statutory double jeopardy provision and if the alleged assault occurred in Georgia a second prosecution is not likely. In the military courts, there are also procedural differences from the state and federal court system with regard to preliminary hearings, plea bargaining and "trial" although in your fiance's situation, as with each of the other jurisdictions, he is entitled to an appeal. First offender treatment is not available in Georgia for the offense of rape, and unfortunately, even if it were, the federal and military courts do not recognize the benefits afforded by Georgia law when it comes to a first offender conviction. At sentencing in a federal district court case a first offender plea is considered a conviction in aggravation of punishment for the federal crime and a first offender plea is also considered a conviction for immigration purposes for green card holders and visiting aliens with regard to exclusion from living in or visiting this country. It seems as though, from the limited amount of information afforded in your question, that your fiance's only recourse at this point since he has been "found guilty" is to pursue the appellate process in the military court. The Jag officer that represented your fiance will best be able to advise you as to the process and time frames for that procedure.