Timmy - I think that no matter what you say, if you write it well and show respect for her beliefs and motives, it will be well received. For example, the first couple of chapters of Crisis of Conscience put the reader completely at ease, because they can see that Ray Franz was very genuine and bore no malice towards the organization. I think if you approach your letter from that angle, and show full sincerity, then whatever you say will be well received, no matter how shocking it may seem.
Try to use the "makeweight scenario" if you can - it certainly helped me. I would also seriously consider mentioning Crisis of Conscience and asking your friend if she could bring herself to read it. I know that sounds ambitious, but like I said, you can say anything so long as you say it in the right way.
By the way, I'm not suggesting that you don't use scriptures, I'm just saying to try not to overwhelm your friend with them. Try to think "would I look each of those scriptures up if she wrote me a letter with that many scriptures in them". Pick the best ones, and make it punchy.