Christians, what is the reasoning behind her doing this?

I agree with you, although I am a Christian myself. But I would never treat a friend this way. Only God can judge and distinguish us from one another by sin, we should not be doing it ourselves. Even if this girl has found God, it doesn't mean she is completely devoid of sin, which makes her no different than her ex-friend. Let he who is without sin cast the first stone...remind her of that verse. We are all equal in God's book. God bless.