I realize it is easy to believe your bf's SO could conceivably embrace poly. But there are some huge differences between your situation and hers. First, you know about her, and probably know some things about her. She probably doesn't feel like a total stranger to you, despite the fact that you are not friends. You are are okay with the fact that he has another lover. Not only does she not know about the relationship between you two, she is not privy to the same kind of small talk about you that you are about her. She is going to feel like her bf has been leading a dual life - a life not only that she doesn't know about, but has been excluded from. The deception is going to feel like rejection. It is going to hurt like a mother. And she is going to feel very disrespected. Rightfully so.
At that juncture, she may choose to leave, relationship over. You two may not have the chance to introduce the concept of poly. Equally as likely, she will give him an ultimatum. Who will he choose?
IF she stays, and IF she doesn't make him choose (highly unlikely that one of those two scenarios won't come to pass), THEN you can worry about introducing the concept of poly.
I am sorry. You two grossly disrespected this woman. While I understand that you didn't start out to hurt her, you have. I won't say that it's impossible to achieve what you want, I will say it is unlikely.