I wouldn't take it upon myself to respond to the messages he sends her. If she asks you what to say, then you can help her w/the reply, but it's up to her to decide whether or not she wants to respond to him. If you do it, it will look like you are not allowing her to do it herself.

All I am advising you to do is get your documentation together and put it all in one place for easy reference when you need to address questions. Keep your eyes and ears open and do not let your guard down from now on. He knows you entirely too well and knows how you panic, get anxious and when you feel threatened, you lash out and then you attempt to try to explain and/or justify things, which digs your hole deeper and deeper. You've got to keep a level head, stay calm and if you get angry and/or frustrated w/him, take a walk, come here or do something else, but whatever you do, don't text, email or call him when you are in that state.

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.