First-

I'm sorry you are losing such a close friend, I hope he wakes up one day and realizes what is going on here.
Second-

Why did he tell her that? I mean yeah he gets an A for honesty about his feelings but if he never planned on acting on them and the two fo you managed a friendship for years why say something now. :duh
Third- I'm sorry she is acting like that I mean if you can't trust then maybe you shouldn't be getting married.
I think you may just have to morn the lose of this friend for now, I don't think this will be permanent. If you try pushing the subject to much it may send her over the edge causing real damage. Sometimes if you go with the flow and wait for the dust to settle you can have a talk and go from there.
Have you tried having any contact with her? I mean I know the friendship was with him but have you maybe invited her over for coffee or tea just the two of you? You may not beable to repair things before the wedding but you can continue to work on them from here on out.
I'm sorry I am no help here I know it because in my world if I ever dreamed of asking a friendship to end atleast one that was there long before I was, I would probably be laughed at. But I respect that.
I think my main advice would be to try and reach out to her if you haven't done so already.
Don't even tell him your plan just go with it that way she thinks it geniune and not some planned scheme between the two of you.
Good Luck and let me know what you decide, I'm sorry you are hurting it is never fun to lose a friend no matter what the situation is.