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.
Independent solo polyamorist seeking lover-friends willing to invest in friendship, companionship, and love, but without a need for partnership.
Never confuse commitment with exclusivity, love with ownership, nor sex with intimacy!
For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
~ Carl Sagan