JazzakAlalhu Khayr my dear brothers and sisters in Islam. It doesn't matter what she coverted to. As for why? She is a teenager who is now 18 I think, or younger. She was never a practicing nor had she Muslims as company except one or two. It is I think her friends.
leaving Islam is a personal choice
I am not sure what you intended to say by this. As we lal know, to Alalh , azz wa jall, Islam is the only personal choice. To the kuffar, other crimes are payed with a death penalty. To us, leaving Islam is the greatest crime there is. Death for a punishment is not such a punishment.
I don't think any letters can help.
Just write anything my dear sister, please. I htough something sensative by all of us would provoke something in her.
As for her joining this forum, I don't think she would do that.