If she had discovered some terrible secret (infidelity, addiction, lies etc) perpetrated by the groom...then of course , awful but understandable.
However , to do this now having had doubts and for no other reason but a change of mind, is just about the most awful, shitty , self absorbed thing to do to another human being. One that believed (until this moment) that they were so loved, they wanted to marry you.
For all those saying 'oh it's SO much better to do this now than later ' I call bullshit. You have obviously never been dumped on your wedding day and had your awful, gut wrenching unhappiness witnessed by everyone of your family and friends. It is one of THE MOST CRUEL things to do to another person and utterly utterly awful. It will have a long term effect on her groom. It will colour every future relationship with mistrust.
NO ONE should be married to someone they don't want to be married to but no one should have the right to humiliate another because of simple precociousness. It is not hard to put on an act, go through with the wedding, and then - in PRIVATE explain to your groom that you don't want to be married and quietly and KINDLY get an annulment. If you love someone enough to want to marry them, then you can forgoe the 'drama' of putting them through the horror of a wedding day change of mind.
It happened to me. 26 yrs ago. I will never ever 'get over' it. I learned to live with it. I have married and been happy. It was a long long unhappy road to get to that point, through no fault of my own.