Aw, that's really sad. :( What a trooper!
I can't really write the letter for you so just speak through the heart. Even without me knowing her or even seeing her- I admire her strength and courage and I think that she needs to be reassured that there is a life in front of her. With so many things that have been crashing down around her, she has been quite resiliant and that is such an admirable trait.
Tell her why she is your best friend and what makes her special. That she can make it through this and that she's come this far. Things will get better soon and you will always be there for her. Write down exactly how you feel. Don't worry about grammar or complete sentences until afterwards and write as if you were talking to her in a conversation. Then you can fix it up afterwards. That always helps me because it isn't being too forced.
If I were you, I would wait to see if you were moving definately before telling her. Afterall, you don't want to worry her for nothing! Or have you already told her? What are the chances of you moving? How far away? I think that your friendship must be really strong and pure which is hard to come by these days. Keep your friendship strong and do keep in contact with her if you do move away. This has brought me close to tears. :P And I'm not even PMSing. LOL
Good Luck to the both of you!