If your child is in the midst of a longtime friendship that is falling apart, the best thing you can do is stand by and be a good listener. It is devastating for us to see our children hurting, but taking sides or intervening is not appropriate, nor will it help. Preadolescents do survive these hurts, especially if they know we are there to listen to their pain. Friends are people who accept us as we are. They listen, they don't needlessly criticize, they back us up when we're right and pick us up when we're down.
All friendships have ups and downs.
People have differences of opinion and even get angry, but they still care for each other. As parents we must help our kids see that one problem doesn't ruin a relationship, but stubbornness might. Middle schoolers have a lot of spats and falling outs, but often the friends are back together again in a short time. When reprimanding, deal only with the precise problem, don't bring in other issues. If the issue is minor, keep things light.