Wow that's pretty selfish and cowardly. I would be very angry and hurt if I heard someone had lied to me about their relationship status at any point in the relationship. I doubt I would want to stay with them. Not only because they lied but because they had been selfish, cowardly and disrespectful. I have a very high standard though when it comes to partners. The bar is very high at this point.
This is not a poly relationship its cheating. That's my opinion. I hope that is clearly stated when this person finds out, because its people like her that make it tough for us honest folks to be proud of who we are.
After a good long wallow in the humble pit of guilt, shame, embarrassmentment and feeling really sorry she has done this. A little kissing ass might help too. This decieved person deserves that at the very least, please know that it can be chalked up to a lesson learned. Honesty is the best policy, right from day one. In my opinion from first encounter. Not second date, not first date, from the first moment that there is a chance it will go further than friendship.
Sorry this sounds firm, but on this topic, I am firm.