Please post the information I previously requested. Without that information, I cannot give you anymore advice than I already have.
Again, you are represented by counsel. You should be directing your questions to you attorney. Your attorney is in the best position to answer them.
I believe the original sentence was legal. You are correct that the minor passenger enhancer makes this a felony conviction. Any conviction for which the possible jail time is over a year = a felony/prison conviction. Under 973.09, the maximum term of probation for a felony is the same as the maximum confinement in prison or 3 years, whichever is greater. The maximum confinement for your fiancee was 2 years. 3 years is greater. She had 3 years of probation available to her.
Of course, this is an argument her attorney would have to make. And, I may be missing something as I do not have all the facts, like your fiancee's attorney.
Again, if probation was not an option, a new sentencing hearing is the correct remedy.