Which should be preferable. To obey the father or to fast?

I am asking this question on the bahalf of a muslim sister. She is 26 years old unmarried girl and fast regularly on Monday and Thrusday. But her father wants to limit it to one day only. If she go against his wishes,is she being disobedient to him ? Is there any hadith that help her to take decision that either she obey to her father or fast regularly ?