I'm not sure anything I said was helpful. After all, no one can or should try to fix feelings like this. She needs to have them, process them, and navigate through them on her own. I shared my opinion that perhaps this friend wishes she could be invited on the trip as well. I explained that even though she was trying to be private about talking about the trip, that in the future she should not share any of it at school. I explained that sometimes people can be hurtful, and that I wish I could say it would be the last time she felt like this but it won't be. I then gave her a few options on how to move forward from this. I said she could take a break from this group since she had other friends she could spend time with for lunch or recess. She could speak with the friend that is upset with her and talk it out, and she could also see the counselor at school who is available for situations just like this. Today will be her first day back and I believe she choose to take a break.
For my daughter and for myself, I remind us both what being a friend really means. I have to give myself this refresher course from time to time. If something comes up with one of my friends I need to check in to see how we can talk it through, and if we can't I need to make sure that this friend still enjoys their time with me, and I with them. All friendships aren't made to last.