The core issue here remains unchanged. If he is committed to polyamory and she is committed to monogamy, they are incompatible at that basic level and should have ended things at that point.
The fact that they remain emotionally entangled with each other is a clear indication that they are each getting something out of continuing the relationship. If they were not, they would have moved on already.
The question you may wish to ask yourself is: What are you getting out of this situation? If the answer is nothing or not much, consider withdrawing until it is resolved.