Apparently one of our mutual friends (of her own volition) contacted him and tried to persuade him to take me back.
I can see how he might suspect that you were, at the very least, encouraging her. This type of misunderstanding is easy.
One thing that often happens in these relationships is that everyone become drama oriented - the "fight" becomes more about winning each battle, and less about finding solutions.
Not knowing the tone of the letter, if this was anyone else and the topic were not so personal, what would we do? We respond with an apology and assurances that we're not behind this.
Does it make sense to send something like this.
Its disarming.
Hey ______,
I can understand how uncomfortable ______ conversation was. I'm sorry. I want you to know that I was not behind it, although I can see why it would appear that way.
I am respecting your space.
__________