Near the end of the film, Matt is on his final day of Lent. To celebrate that evening he and Erica plan to have a romantic evening together. However unbeknown to Matt, Nicole learned of the online bet that Matt's co-workers set up. She places a large bet, then takes advantage of Matt when he is sleeping handcuffed to his bed, moments before his 40 days of Lent are up. There are two reasons Nicole did this.
1.) As stated above, Nicole found out about the betting pool that Matt's co-workers set up, Nicole wanted to collect on the large bet she placed.
2.) The more obvious reason, She wanted to humiliate Matt, Nicole became single and wanted Matt back, however Matt now is able to resist, this makes Nicole jealous and angry, so she planned to humiliate him in the most hurtful way possible, to rape him, make his vow void, and to make him take the blame for it, since in the end Matt feels guilty about not being able to control himself, which in reality is not his fault, and has to apologize as if he had casual sex with Nicole.