So, she wanted a relationship with you and thought your wife didn't know.
But did she actually try to break you two? Did she encourage you to leave your wife?
The way I see it, a cowgirl starts a relationship with the intent to "have you for herself". Is that really what she did?
If she wanted a relationship with you and though you were trying to cheat (since you were hitting on her while being married) I think I would give her some slack, provided you told her you were interested and didn't tell her you were poly.
Because in that case, it seems to me she didn't realise there was an option to be open about it. In that case, her thinking might have been "he's obviously going to cheat on his wife either way, and I like him, so I don't want to reject him".
It would even be possible that she would have preferred being open about it, but didn't know it was an option.
If, however, she approached you first with advances, it's a tiny bit different. While she might still not have known poly was an option, I believe more people would refrain from hitting on someone they like who is married. I mean, it seems to me that case A (someone you like hits on you, he's married, do you go along or not?) and case B (someone you like is married, do you hit on him or not?) are different. I'm aware of poly, so in case A I would say "poly or nothing", but if I didn't know about poly, it might be hard to resist: his wife is more his responsibility than mine, he's already made that call (to cheat on her), and I really like him.
I would really need to know the circumstances. Either way, how long has it been? If she really likes you and it hasn't been long, she probably isn't over you and starting a friendship with the intent of it staying a friendship forever might not be the best idea. It's possible that she's hoping it will turn into more, for instance. If that's the case, you'll have to make sure she fully understand that a relationship isn't an option anymore at this point.