Make sure your teen knows that, just because a friend moved away, he's not gone forever. Thanks to the incredible power of social media and technology such as Skype, your teen and his faraway friend can still keep in contact, chat, and share news, photos and videos. Have them plan a time each week that they can spend keeping in touch long-distance style. If possible, arrange a visit to the friend's new town a few months after the move. This can serve as a transition period, with the anticipation of seeing the friend again in a given amount of time.
Your teen may be so overwhelmed with the loss of her friend that she may not be able to see beyond it. Remind her of all her other great friends, and encourage her to reconnect and have some fun with them.
Wallowing in her room is not going to help her get past the pain of losing her friend. Help her get out of the house. Introduce her to a new activity or sign her up for a new sport in which she may be interested -- anything to keep her from dwelling on the loss of her fave friend. This will help her make new friends with common interests, too.
If you ask how your teen is doing and he just shrugs, "fine," don't let it go. While you don't want to push your child to open up about feelings before he's ready, working through the pain is important for him. Remind him that you are there for him if he wants to talk and that whatever you talk about is confidential. Share with him about a time when you felt the loss of a friend, and be patient. If you notice warning signs that he might be falling into depression, seek medical advice right away. Withdrawal from friends/family, lack of enthusiasm and motivation, fatigue, trouble concentrating and loss of interest in activities can signal depression.
If your teen's sadness over the loss of the friend lingers, don't take it lightly. Be persistent, even if your teen shuts you out. Tell your teen that she can talk to you openly without fear of judgement or punishment.