I can ASSume with the best of 'em, Bets, and I'll take a crack at what you've said. I'm ASSuming, from what I know of S., that his letter will be warm and sweet and heartfelt, and that he will say he misses talking to her, and probably NOT end the communication, and since he is saying he wants to do it because it's "the way to treat a friend," that he is in actuality responding to a postcard from her, that a communication from him will just re-open the lines of communication.
Here's more ASSumptions: He will be vague, if he says anything at all, about being in a R, and if he tells her we are back together, he could very likely say that things are "a mess," terminology he's used before. So, she is left thinking
1. He misses me 2. His R is on the rocks 3. I can be understanding and connect with him 4. Our 4x/week-plus communication can resume.
She will make a tentative phone call, they will talk for three hours in the middle of the night, and he will lament having lost this most special of "friendships."
I've been back and forth on these issues with H2H all day, and I wrote to her:
I do think it’s appropriate for him to think about what his intentions ARE before he sends out a letter, which knowing him will be really tender and heartfelt and probably open up ALL kinds of communications lines between them. Remember, SHE is the one who understands him, and I don’t. They really “connect,” as he puts it, and we don’t. So this is really dangerous waters, especially when we are so fragile. I need also to ask him to put off the letter until we are on more solid ground.
Which also answers the question, in a way, "What's she got that I don't got?"