Her mother should be ashamed for laying such news on a 16-year-old. Quite unfair.
Also, why did she give her daughter a boy’s name?
Brett is an English surname.
Many people, male and female, are given family surnames as first names.
Kelly, Morgan, Riley, Ashley (of various spellings), and many others are names first given to females AND males because they were family/ancestral names; and over time they became common female names.