M.R.
Yes, I know you don't want moms with young kids to respond but I also come with the experience of working with high school aged kids for 15 years and have had to deal with a LOT of crying teen girls and drama.
I know that you meant well, but please...keep this conversation between you and your daughter and let YOUR DAUGHTER work this one out. Now not only is she the one who was intentionally and hurtfully left out of the movie, she is also the poor, mortified girl whose mommy called and told on the ring leader. Yikes! Don't get me wrong; I don't think what the girls did was very nice (excluded others and lied about their plans) but this sort of thing happens and the best thing you can do as a parent is to help your daughter learn to negotiate through times like this.
One of the best things you can do is sit down with your daughter and ask her, even though they've been friends for a long time, does she really want to continue to be friends with people who are going to be hurtful and exclude her? Are those really 'true friends'? Help her role play with you as practice before talking to the girls, using questions such as "I thought we were friends. Have I done something wrong to upset you or make you not want to be friends with me anymore?" Here's the tough part though - over time friendships change and evolve, and she might not be happy with the answer she gets, but at least it gives her a chance to have a conversation with the girls and find out what's going on.
Maybe this friendship is salvageable and maybe it isn't, but at 15 years old she needs to be the one to confront her friends and negotiate. Your role is to be there to support her and help her figure out how SHE is going to handle this situation.