It really hurts when the people we care about move on...life tends to move people in and out of our lives when we least expect it....your friend may have felt safer announcing this in a group setting, she may have been afraid of your reaction, afraid she wouldn't have been able to get it out without breaking down etc. ... safety in numbers....
I know you'll miss her terribly but we have to let people go...I lost a dear friend years ago from a move and I still wonder how she's doing....but I know she did it to better her life and it had nothing to do with our friendship...try not to take it personally she's only doing it for reasons she feels are best her and her BF...hang in there...