Something tells me that she doesn't understand the cycle here. She wants more out of this whole threesome than she's getting. Sounds like she agreed to it to get close to him, but now she wants him for herself...or she wants more of him than you get to make herself feel more important - it's could be a power thing. From where I sit, I only see three options here... sit the both of them down and talk it out and try to work it out, just ignore it and put up with it (which i wouldn't recommend choosing), or leaving the relationship. Hope this helps, good luck!