What Cat said.

There is no "friendly" flirting. If you are both single, it's fine. If either or both are in a relationship (committed or otherwise*) then it's wrong.

And, can I just say how much I dislike women who will actively make a play for another woman's husband/partner/boyfriend/fiance just because they can.

* I say "committeed or otherwise" as I was once the person who didn't realise that my partner had moved on.... pity the narky soandso forgot to tell me about it first.