Change father's name in transfer certificate.

11 years back my sister went with a guy. But they were not legally married. After the birth of a daughter, the guy left her. While joining school, in her daughter’s admission form, in place of father's name, she gave an anonymous name because she doesn't want to put the persons name in her daughter's admission form who left them both without any reason. Few months back my sister got married with a guy legally and she want to shift to her husband's place. Now she want to change father’s name in daughter’s transfer certificate. She doesn't have any document regarding the anonymous father name. What can we do legally to change the father's name in her daughter's Transfer certificate in the present school.