Here's how I would frame her point of view. If Christianity is not true, then Western society will be absorbed by another more powerful story. And we will become part of that story. So, we ought to be having a conversation about these differing stories.

What I would add is that the true story is ultimately the strongest one. When Christianity was nothing, it took over the Roman empire with zero power and Rome having all the power. This was empowered by the Holy Spirit and embodied by Christians following the law of Christ (loving others as Christ loves us).