He was clearly feeling sad and ashamed and said 'no' to that. Then she said, "Yes, had you not opened the door, our bunny would still be alive right now!"
There is a lot wrong with this. First of all, this boy is only 7 and autistic and he loved the rabbit. The idea that she is making him feel guilty for it dying is sickening.
Not only that, it's shockingly poor parenting. Now what if he goes to bed thinking he's a "bad person" and that takes root in his psyche at 7. Not good.
It's such a tough position. On one hand, I want to be able to stay in his life to provide comfort and support... .but on the other... .I'll never be able to have a relationship with him that doesn't involve her inserting herself between us. I don't know if my trying to contact him is doing more harm than good, since she appears to use him to be able to stay in touch with me.
That is a tough position thrown. It would be easier if he was older, but it is what it is. Are you getting to the point where you're going to make a decision?