You are correct that in the state of California, a 2nd degree robbery is punishable by up to 5 years. However since he had a prior robbery offense, this is considered an enhancing factor which can then be used to add additional years on to the sentence.
However, based on the information you provided above about the alleged incident in this case, it sounds as though perhaps your fiance was not properly represented by his lawyer.
In that case, he would be eligible to file a Post Conviction Motion for Ineffective Assistance of Counsel.
To have an Ineffective Assistance of Counsel motion granted, you must be able to prove that the attorney's representation in his trial was incompetent in some way, and that this incompetence prejudiced your fiance's case.
If it's found to be the cased that the attorney's representation was deficient in some way, which hurt your fiance's case, for example by failing to present evidence is his defense, then the judge may reopen the case and even adjust his sentencing.
This is something that you would need an attorney to draft, file, and argue in court for you, but you will want to retain a different lawyer than the one who represented him at trial in order to do so.
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