Since men are in positions of power in quite a large part of the world (the equations are changing now, but still)---they can do no wrong.
So women who make advances/ flirt/ have multiple relationships are cheapened, whereas men with the same behavior pattern are romanticized--which is why the whole M &B equation of experienced man teaching virginal woman the ropes of love/lovemaking is so popular and embedded in popular culture. An experienced woman initiating an inexperienced man would be called abuse, of course!
For me, I don't find Khushi's interaction with NK 'flirting'--even the one where she models bangles for him. To her, she has found a way to annoy Arnav, and is using it to divert Arnav's intense scrutiny from her to NK. The flirting part came under discussion because it inadvertently yet directly 'invited' the 'male gaze' of NK. If NK had platonic feelings in mind, his gaze would not have been attracted (no matter how deeply-cut her kameez), and Khushi wouldn't have been perceived to be 'flirting'!
That said, even if she did flirt, I'd say she would be perfectly within her rights--as long as she did so with full awareness of the consequences (given the current social environment and existing double standards), and confidence in handling them.