What should I do with my wife who delays the Fajr prayer past its time despite her commitment to the other prayers?

You must wake your wife for Fajr prayer and remind her of the seriousness of neglecting prayer and what Allah has prepared for those who are negligent. You can also give her books and tapes on the subject, take her to attend lectures, and try to connect her with righteous and pious women who can help her in goodness and prayer.