Believe it or not, I think it is possible for one to accept at least the cultural and historical aspects of postmodernism without rejecting Christianity. Postmodernism is more of a mindset than a philosophy or religion. In matters of culture and music, I actually like the postmodern idea, as it sees diversity as strength, not weakness. For this reason, I suggest you not come across too defensive toward your friends attraction to postmodernism. Yet, the postmodern tendency to say all ideas are equally valid is absolutely caustic to Christianity. Besides, it is an incoherent philosophy. How can inherently contradictory ideas both be true? If the Christian idea of God is true, then the Hindu concept of Brahman is absolutely, fundamentally false. If Jesus is who he said he is, then Muhammad was a liar and Buddhism, although perhaps a nice philosophy, is fundamentally untrue as a religion. Postmodernism is confusing at best in the area of morals and, in the end, it denies the existence of truth. In this sense, it is self-contradictory, because if the truth of Postmodernism is true (that there is no absolute truth), the Postmodernism itself is not absolutely true.
In my opinion, you will do better to point your friend toward Jesus, his life, his miracles, his fulfillment of prophecy, his ministry of compassion and his claims about himselfm rather than trying to “disprove” or talk her out of Postmodernism. You can ask your friend to read the Book of John and to discuss with you the miracles and the claims. You can then ask her what she believes about Jesus. If she has common sense, she will see that the postmodern idea about Jesus is irrational. My adive, then, is to point her toward Jesus. I am attaching a lesson I did recently which is an attempt to do just that. Claims of Jesus
I hope it helps. You will find plenty of scriptures there which will challenge her idea that all truth is more or less equally valid. In this power point I prove that if Jesus is who he said he is, then Christianity is the only completely valid source of truth in the area of religion.