I dunno, what do you define as "flirting"? Because me and a few of my female friends always "fake flirt", you might say. We might be working on homework together and one of them will say "Hey, I bet you wouldn't mind if we took a break from this. So... want a good time?", or I might come up behind one of them and say "Damn girl, I couldn't help noticing your sexy ass!". One time one of them was sitting beside me in Bio class, and she decided to grab my thigh and slowly work her way up. Kind of awkward...
It's all in good fun, and we obviously aren't serious, but a stranger witnessing it might think it's real flirting. So, it all depends on the people and context of the situation.
If it's not fake flirting, then call her out on it. One day when she flirts, ask her if she feels anything for you, and if not, why is she flirting. Remember that it could just be her nature, not everyone flirts only with people they're interested in.