Print the letter, sign it, address it. Then put it in your desk drawer for at least one month. If you still want to send it then, go ahead.
IMHO sending this letter will drive you friend further into the Borg mind. You are giving them 'something to do', a righteous call to action, a test, an opportunity for him to demonstrate his faith. Your letter will probably be passed direct to the elders and your friend will get that warm fuzzy feeling of 'doing the right thing'. It may well strengthen their faith, avoiding a nasty apostate attempting to tempt them away is no mean feat!
The advice to perhaps meet and have a coffee would be better. Don't bring up any JW related issues until they ask and then only answer the questions they asked without launching into a tirade!