Sounds like duress to me.
"The prenup was signed the day before her wedding, against her will, but she signed as to avoid a cancellation of the wedding."
A will or prenup signed "under duress" is invalid. The issue is whether she can prove it was signed "under duress". Only a qualified attorney can advise whether there's enough evidence to prove the prenup was signed "under duress".
Here are two key facts relevant to this situation:
1. It was signed the day before her wedding (even the number of hours before the wedding is relevant here), and
2. It was signed "against her will" .
Fact 1 **suggests** that the prenup was signed under duress.
Fact 2 is an allegation that needs to be backed up by real, admissible proof. Did she tell anyone she was signing it "against her will" on the day (and even the moment) when she signed it? Did she say anything -- to anyone -- (other than her husband, of course) that would lead a reasonably prudent person to believe she was, in fact, signing it "against her will"? Did anyone observe her that day? Did anyone witness her signing it, and notice her emotional state? Can anyone testify she was upset? You see where I'm going with all this, I'm sure.
I am not an attorney, I'm a paralegal, so the opinion I offer is unqualified. Only an attorney can give her one that is qualified.
As one of the other posters said.... she was married for so short a time, I'm not sure it matters whether the prenup is valid or not. But you asked, so I answered.