I totally get where you are coming from. You are in a very difficult situation. As you have opened your heart to feeling compassion for this woman and to recognize your part in hurting her, you now feel a sense of wanting to protect her from further harm.
That makes perfect sense to me. Unfortunately, as you already know there is no way you can do that. If you did tell her, she's likely not going to believe you More than one AP has tried to muddy the waters out of vindictiveness and she has no reason to believe you have good intentions even though you do. At this point the best thing you can do for her is what you are already doing - staying out of the situation all together. Make no more trouble for her, have no contact with him and focus on being the best you that you can be. Take heart in the fact that now that she's aware of this betrayal, she will be far, far more alert to any additional ones. I think one thing all of us BS can agree on is that your spidey senses become VERY sharp after DD. And things you told yourself that you were "blowing out of proportion" in the past don't get brushed off so easily any more. So it is unlikely that he will get a free ride in the future. But at the end of the day, you can['t save her. She will have to do that herself. You should know that better than most. You can only be sure that YOU aren't the one hurting her - and you are already doing that. So try to let go of their story, his and hers. Focus on the only one you have 100% control over - your own. You are on the right track now. You are focused on healing your own past trauma and making choices that are good for you. Be passionate about that, Daydream about the life you are building. Get so focused on you that you forget about them.
BS - Female
Married 27 years, one adult child DD May 2016 “When we are no longer able to change a situation, we are challenged to change ourselves.” - V Frankl