I'm not sure what you're asking, but I think the question is "Will my public defender be able to accomplish this?"
I don't know any lawyers who guarantee outcomes. There's too many unpredictable factors (i.e. what the judge, jury, or prosecutors will do). And public defenders are usually pretty good. But it's safe to say that private attorneys are better -- not because of skill-level, but because of time committment. A typical public defender is massively overworked, as they deal with thousands of cases a year. A privately-hired criminal defense attorney can usually manage caseload enough to devote more time to fewer clients.
So, note that hiring a private attorney will NOT guarantee an outcome. And it has the disadvantage of being "not free." But it may very well secure a better outcome, or a better likelihood of the outcome you want, so it's worth investigating. I'd use avvo.com's search feature to find a criminal defense attorney in your area who does free consultations, and call them to take advantage of it.
Good luck, to both you and your fiancee.
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Like the previous attorney, it is not crystal clear to me what you are asking. Public defenders, like any other group of people, can vary greatly in their dedication, competence, and experience. Some are great and some are not. The two most common drawbacks to a public defender are that you don't get to choose your public defender and, therefore, have no control if you get a great one or not. Secondly, Public defenders have a high case load and are in court much of the time, leaving them less time to work on their cases than most private attorneys.
As far as the prosecutor waiting to charge him, it is possible and I have seen prosecutors do exactly that, but there is no guarantee that is what is going on. He should talk to an attorney about his situation in detail to make sure he is doing all he can to protect himself. He might have options available to him to prevent the prosecutor from waiting until he gets out to file new charges, but an attorney would have to discuss any such possibility with him after finding out about all the specific facts of his situation.
This answer represents this attorney's opinion regarding a general question. All cases and situations are very fact specific and require a thorough knowledge of all facts, circumstances, and evidence to give proper advice. The opinion expressed here is for general information purposes and is not meant to be taken as specific advice or as forming any attorney-client relationship. I highly recommend that you discuss your situation with an experienced attorney in this area to obtain specific advice on your situation.
Public Defenders like all attorneys are not created equal. There are very good and bad private attorneys and the same is true for public defenders office. It is extremely difficult to make any recommendation about the possibility of time served on any new charge. If the case is weak for the state or you have a good defense issue it is more likely to be worked out in the defendant's favor.