You're an asshole. She's my fiance and there is a WE in this. I'm the first person she has ever told! Who the hell do you think you are you insensitive bastard!!?
YOU do not seem to understand - YOU are a legal NON-entity in this! YOU are not a witness nor in any way involved. Your only part in this is to be emotionally supportive.
Disagreeable was correct - there is no "WE" in this. As far as the legal aspect goes, there is her, the suspect, and any potential witnesses. So, tone it back a few notches. Such language will NOT be tolerated!
Legally, this will be nearly impossible to prosecute. If the first she told was 16 years after the fact, and there is no forensic or medical evidence to corroborate the allegations, absent a confession or an admission by the suspect to some other credible third party, a successful prosecution is highly unlikely. In fact, ANY prosecution is unlikely. But, she should still report it.
Now, if you want to help your fiancee, then be there for her, make sure she gets into therapy, and help her work through this. But, do not count on any retribution from the justice system because it is simply not likely to happen. If he can be prosecuted and is, great! If not, she will steal need healing.