A lot of people vow to keep themselves before marriage for whatever reasons, religious or personal. I once wanted to save myself until marriage, but gave up shortly after high school because it just wasn't practical.

Have a talk with her about her feelings about it. Ask why she was keeping her virginity. Changing your mind is not a bad thing, as long as the reasons are right. It doesn't make sense to hold to a committment if you don't believe in it anymore.