Middle school is rough, and relationships are often tested. Even if your tween has a lot of friends, it's likely that at some point he or she will feel left out by a good friend or the group as a whole.
It's hard.
But you can help your tween through these tough patches. Listen when your child confides that there's a problem with friends, and offer up a little bit of advice. But be sure to give your child the chance to work through the issues by himself. That's how kids learn, and many problems are temporary -- a big deal one day, gone the next.
If your child is really struggling, you might consider finding activities outside of school that give her a chance to socialize with other children. Also, open your home to your child's friends, you'll learn a lot about their relationships, and that can help you offer advice, when the time comes.