Adults don’t know how to grieve, you can bet teens don’t either. Most are completely baffled what it’s all about. And most think “being strong” means denying or pushing away one’s feelings.
The Dougy Center, a foundation for grieving children, has many resources for youth of all ages including teenagers.
Here are links to resources on that website and this one:
Grief Resources for 12-18 year olds (Dougy.org)
Your friend is grieving. How to support them (Dougy.org)
Tips for grieving teens (Dougy.org)