They are talking about her parents. She’s Catholic
Going to a Catholic school doesn't make one a Catholic -- but in Islam -- being born into a Muslim family (and it's a Muslim family) makes all the kids Muslim. That's the way it works in Islam -- something that many don't know about ... :-)
Try to leave and there's a religious imperative (for any adherent) in Islam to kill you ... and all it takes is one killer out of over a billion of those crazy adherents to Islam to kill a defecter from Islam.
They would kill her because she is no longer a Muslim, not because she still is.
This is America. Obviously, her parents weren’t all that devout - like hordes of other nominal muslims. And in America you can choose your religion. I know two Iranians who converted to christianity. Happens all the time.