This situation sends up all kinds of red flags for me.
She's getting married, but this hasn't been communicated to you before now?
To me, poly is about being able to let relationships evolve to whatever they may become. While I understand rules play a big part in a relationship, I have to wonder how comfortable he truely is with polyamory if he's stipulating only relationships with women are okay.
I also noticed a lack of feeling in your posts, with one glaring exception. You mentioned fear of losing a friend and partner. Do you feel this because you think she may be committing to monogamy with him? If so, I think you and her need to be realistic about what your relationship means to each of you. Perhaps she's got a different set of values that differ from yours, which could spell trouble even without the pending marriage.
Anyway, good luck. I know it's not an easy situation to be in. Emotions run high, especially when love is involved.