I personally vote for sending the letter now, or as soon as you feel the time is right for you. You're not asking for anything or demanding a response or a renewed relationship
, you're not askeing her to chose between you and her son, you're expressing your heartfelt thanks.
As I understand it, she asked you to cease contact as she felt that, at the time, this would benefit her son's mental state. However, correct me if I'm wrong but did she ever say that you were never to contact her again
? If she did, that's a different matter. But if not, well as PPs said, he is apparently doing better, dating
again, and it's been a year. If the relationship
between you and her was as close as you say, I cannot believe she doesn't think of you, and regret the situation that came about as a result of her son's illness.
I say do it, and the best of luck!