It's made up. If somebody calls themselves an agnostic Jew, they are generally saying they are an agnostic of Jewish cultural/ethnic/racial background. That's all. "Jewish" has multiple meanings - it can refer to a follower of the religion of Judaism, a person of Jewish/Hebrew ethnic or racial heritage, or a person raised in Jewish culture/traditions. Unless she has some Jewish ethnic background, this makes no sense. But do you expect them to make sense?