I'm No Lawyer But....
My neices husband had a simple possession drug beef here in CA. They gave him what they call "diversion", which diverts one out of the court system. He had to take drug classes, pee in a bottle every so often, etc.
At any rate, when he/anyone applies for diversion, a "Not Guilty" plea is also entered. This was several or more years ago. He now passes the CA DOJ background checks just fine. From what I understand, his arrest shows up on computer checks, but that's all that shows up. No conviction statistics are listed, as if the charge were dismissed or found not guilty, which is how he plead in court.
What I'm getting at is he was never pronounced "GUILTY" by a judge or the court or himself.
If the circumstances for the fiance are the same, (e.g. she was never pronounced guilty or plead guilty), I'd say she is fine. If she was ever pronounced guilty and convicted of a felony, she won't be able to truthfully pass the 4473 requirements and will be denied.
If you are not sure, see the DA who handled the case. He will tell you for sure if there was a conviction.