Proverbs 22:6. She is still young so I believe her mind could change. I would ask her about what type of future she would like to have (i.e. kids, wedding day) and see if she's really thought it out. If she does eventually want kids, I'd explain to her that she' be denying her kids of everything her father has done for her. I think you've done a great job of being understanding. Keep praying and hope that her heart changes.