Aww I can sort of relate to how you feel. One of my sorority sisters who i considered one of my closest friends (despite falling out of touch, but its only been a few months, since i had been having some hard times) got married this past weekend and I wasnt even invited :-(
A friend asked if I was going to confront her about it, which I think would be ridiculously selfish...to ruin even one shred of memory about her wedding with a confrontation. But I was upset and confused since we hadnt argued or had a negative falling out or anything, AND other sisters were invited.
For your friends sake, dont confront her