If I were in your position, I would write them a letter and tell them your thoughts. tell them you want to get to the bottom of this issue. That their fears and concerns are valid but not caused by you but by something/someone else. That you want to help them see through the terrible things that were so terrible, that it caused them to cut you out of their life. Tell them you want this because you are afraid, too. You are scared of what this has done to your relatinship. You are scared because it makes you not be able to recognize your person right now, either. You also want to see them as you know them, not as their enemy.

If this isn't more about you defending yourself, then you would do well to step up and show your support and determination to get through this. It sounds traumatizing, what ever it may be. Trauma is tricky and makes people do and believe and think some pretty far out stuff. If you can get past the 'their paranoid and off their rocker and making accusations about me" and into the "I see how scared you are and what you are saying is very scary and I am scared for you. But I am not responsible for these -whatever accusations- and I know you don't believe me but that doesn't matter because I don't need you to. I want to help you figure this out. Please let me support you through this by not shutting me out". Or something along those lines.

There is a lot of power in valuing and acknowledging a person's feelings. Even if we don't believe or understand, that does not make the person's experience invalid nor give us license to dictate how they should be feeling, according to our morals and beliefs. We need to let them do things according to their morals and beliefs because it won't work, any other way.

Find a way to be supportive if you want to. if you really truly want to, you will find a way.