You are right to be concerned about your child/teen's friendships.

Friends are secondary attachment figures-- in other words, practice for adult relationships. What kids learn outside your home about how to interact with peers, how to resolve conflict, how to ask for what they need, how much they can or should trust other people-- all combine to create habits of relating that they will carry through adulthood.

Kids who have healthy friendships feel good about themselves, are more confident, do better in school and are more likely to grow into well-adjusted adults. Solitude can lead to isolation, depression and discipline problems. Kids who have unhealthy or toxic friendships, however, also show signs of emotional distress, depression and anxiety. 

Your child's own behavior may be the reason for friendship issues.

• Bossy, disruptive, or self-centered kids annoy their classmates. It's not fun to play with those who don't share or follow rules, or who explode when things don't go their way. 
• Shyness and insecurity can also limit a child's friendship circle, as can being different from others or moving into a new class or school.
• Whatever the cause, friendship worries signal a need for extra parental support and understanding.

Here are some questions to ask:

• Does your child/teen behave differently after spending time with a friend? 
• If your child/teen often seems to have problems with friends, what patterns do you see in these relationships?
• How do fights with their friends usually start? How does your child/teen react to conflict?
• Is your child often in a caregiving role with his/her friends?
• How well does your child/teen assert his/her needs in friendships?
• What have you taught your child about friendship through your own friendships?

What you can do:

• Talk to your child/teen about characteristics of healthy vs. unhealthy friendships.
• Model what being a good friend looks like and talk about why/how you are choosing to be a good friend.
• Compliment your child/teen when you see him or her being a good friend, so they begin to recognize what that is.
• Ask your child/teen who is a good friend to them and how.

Learn more here