Boundaries. Boundaries. Boundaries. You are the only one in this situation with a hope of establishing boundaries, so you must be the enforcer to protect yourself.
I used to work for a divorce mediator. Every single case that came through the door, I started off sympathetic to one party, and when I heard the whole story/read all the paperwork, I was shocked that the opposite party had not murdered the first one years ago. I am extremely cynical. There are a lot of sick marriages out there, and there are a lot of horrid people out there, but nobody knows what really goes on in a marriage except the two people in it.
Annie has no boundaries at all. What you may fail to see, is that Paul has very few/very weak ones himself. He married this woman for a reason, because he was attracted to her. DO NOT FORGET THIS. Unless she had a total personality transplant since the wedding, HE CHOSE THIS. I am not saying he does not have a good reason for divorce, I am saying that as healthy as he may look by comparison, he picked sick.
Get out of this. Stay out of this. Don't let either of them talk to you about it at all. You need to maintain a working relationship with Paul, which gives you a perfect excuse to keep throwing your hands up and saying, "TMI". If Annie does manage to speak to you in person, just tell her flatly that you have to work with Paul and you cannot get involved.
If you want to be extra kind and you have the phone number of a counsellor/religious leader to refer them to, great, hand it over and walk away. Do not engage. If these were the kind of people who did have boundaries, it would be great to try to support them. That is not the case here.