From a position of strength. You don't necessarily have to force your Christian music on her, although you should fully expect her as your friend to meet you halfway. I've had good friends who said the exact same things to me "You're god (little g) isn't even real!" Such things were said over the internet, but I recall grinning to myself. I know very well my God is real, only he just can't see it. So I held to confidence more than ever before, without allowing myself to get into strife with him or be resentful. Must have been kind of infuriating for him, but then I knew that was just part of the deal he was just gonna have to deal with.
I was constantly in contact with that friend through a website that went into severe website problems which haven't resolved yet, and that's been two months ago now. Might have been God's will since He wanted me involved over here more anyway. But I think about my friend now and what he might think of never getting to talk with me again. He knows how together I am, which witnesses to people more than they're willing to admit, even if they don't agree with you.
Show your friend how kind, considerate, caring, and faithful you can be to her, but with absolute and unwavering confidence in the Living God, one that cannot be shaken. If she cannot tolerate that, then you may have no choice but to let her go, and pray for her to return to you in acceptance. When my friend spoke those words to me it was as part of a seriously heated argument where he said a whole host of other ugly and insulting things, yet within a few weeks he was back to responding to my posts politely again, and agreeing with me on things. Why? Because I kept my cool and never insulted him back, yet unswervingly held my ground.
The same will probably work for you.
Best wishes,
HiH
