A Christian coworker: what should I do?

"Somewhat" immature? Were those hate-filled e-mails sent on your work computer? If so, take them to HR and file a complaint against her. If she insists on being a childish idiot playing high school popularity contests, you don't need her. Who wants a 'friend' who doesn't respect you and turns on you when you disagree with her about something? Cut her loose.