Your Facebook friend has presented a pretty low hanging fruit argument there I would reply with something along the lines of:
"Whilst it's tempting to view female empowerment through the lens of equality and things being 'fair'. Your point is quite attractive. We don't want to treat people differently would we? Of course not!
However unpack the idea just a little and it only works when the surface of the issue is not so much even scratched, but barely tickled. When we look at the context of flirting and apply more in depth criteria it becomes a very different picture indeed. Even if our primary objective is to reach peak fairness.
When a man interacts with a woman there are different forces at play for both. Different thoughts going on under the hood. We could analyse this till the cows come home but let's restrict ourselves to a main headline just to make the case. When a woman interacts with a man she is having to weigh up the historic and present contexts of men and women's interaction. This is a context of violence, both sexual and just plain old fashioned violence. This is exacerbated by the average disparity of size and strength between the sexes.
If any woman weighs all this up and finds a man she is comfortable to 'flirt' with then that's cool. Everything can sail off to the land of mutual and reciprocal interaction. A man making the same choice only really needs to ask one question? Am I attracted to this woman?
Now given this is it fair to hold men and women to the precise same standards? Well yes I suppose it is, but only if you are prepared to don massive blinkers and ignore all the hoops many women need to go through in order to get to the same place as men naturally inhabit.
If fairness is our goal surely the fair course is rather than trumpet namalt, we examine where women are, what they have to contend with, and rather than say "I'd never harass a woman, I'm one of the good things ones!" men take stock and realise that the inequalities women face befoul and contaminate even healthy everyday interactions.
The problem in reality affects us all, and leaving women to deal with a problem not even of their own making all on their own is neither fair nor equal.