The prosecutor has no bearing on whether one is eligible for a CHL. Either the court will grant the restoration of rights/sealing of the record or it won't. The prosecutor has the statutory right to object to your fiancée's petition but the court is free to weigh his objection as it sees fit. Once the court disposes of the petition that ends the prosecutor's legal interest in the matter.
Having said that I think the court could probably say it will grant the petition on the condition that he does not obtain a CHL. If he goes ahead and does so it would be contempt of court. You can say hey that's BS, but the court has full discretion on whether or not to grant his petition so options are kind of limited.
I don't know the particulars of your fiancée's case or the politics of the jurisdiction involved, but it seems to me that 24 years of leading a law abiding life (hopefully not even traffic tickets) is a pretty strong track record to point to. Particularly if the circumstances of the conviction weren't major, e.g. didn't cause serious injuries and get pled down from a felony, etc. For the prosecutor's sake he'd better have some strong, articulable objections beyond "I don't like this."
At the end of the day the court makes the call, and props to you for having an attorney involved rather than proceeding pro se.
As an aside, it's my personal opinion that somebody looking to have their record sealed should never mention firearms ownership/hunting/obtaining a CHL as a reason. Those are sure to raise red flags along the way. There's far more politically correct reasons that can be offered. If, after the record is sealed, one were to choose to obtain firearms or get a CHL, well that's their business. Again I don't know the particulars of your fiancée's pleading but to somebody coming across this thread later it's something to consider.
Never ask if you can carry at a non-posted place, but always ask why you cannot at one that is!
Just because they offer call ahead seating doesn't mean you call ahead for carry permission.