Stop focusing on this other gf. Just manage your relationship with him; you can't control or manage anyone else. She has a right to live her life and express herself in relationships as she sees fit, as does your bf, and if your bf likes her, wants to be with her, and thinks it will work between them, why should you want it any different? It is still possible to have a big poly tribe if one or several of you have outside relationships with solo people who aren't into the communal thing. Relax and focus on finding satisfaction in your relationships, and strive not to indulge in the need to be in control.
The world opens up... when you do.
"Oh, oh, can't you see? Love is the drug for me." ~Bryan Ferry
"Love and the self are one . . ." ~Leo Buscaglia "