As a mono in a poly relationship, I'm wondering if she's been mono all along. I know I fell in love with a poly guy, but when I found someone else, I totally broke his heart and left for the new person, which was a jerk move, similar to what she seems to be doing. I wonder how much experience has she had in dating, poly or otherwise, because I had very little, and I thought because my partner was poly, I had to be as well. Now, having gotten to know myself a little better, it kills me that I did that to him.
I have to say, I would have trouble being a secondary as a mono person. I am the primary in my current relationship, and I am mostly very happy.
Just giving you an alternate perspective. It sucks, and I hope you guys can find a good balance.