“Dated,” as in used to date? Or are you interested in seeing her again? And was it just dating, or did it lead to physical intimacy? Those are questions you’ll want to answer.
If you find yourself in a similar situation in the future, meeting someone interesting who is divorced and Catholic, a casual way of bringing up the question of whether the person’s marriage was found to be null is saying something like, “I’ve heard that the annulment process is pretty exhaustive, how did you find it?” The answer to that question will tell you if the person is actually available for dating, or still married in the eyes of the Church. At least it will spare you wasting your time.