It would be best for her to sit down and discuss this with her pastor. Then all information and questions can be hashed out properly.
But just based on the information given:
Since the annulment was granted after the marriage to your father, that marriage was probably done without the bishop’s dispensation to marry outside the Church. If so then the Church would require either a convalidation of the marriage, or the marriage to be performed in the Church. Again, these matters are VERY hard to accurately hash out online. In person with her pastor is best. I would recommend setting up an appointment to discuss these matters fully.
I would suggest being delicate about the matter, and encourage the face to face with the priest. If presented the wrong way, people can take this information as an insult to them and their marriage. It has to be handled gently. And be REAL careful when you discuss with her.
Parents sometimes can react negatively if this information is presented to them by their children. It’s called the Powdered Butt Syndrome. Once you’ve powdered someone’s butt (i.e. changed their diapers), you no longer want their opinion on many personal matters. Tread gently.