I think the answer to your question is that you should let your friend believe that she is right. I feel it’s not that she does not believe you, but that she does not have the courage to believe you.
Her world will shatter if that one belief she holds on to is crumbled. That unconditional love seems to be her lifeline. After realising that she is being adamant you should understand that the issue is far more deeper than a simple debate.
