Well, one way is to take the central idea or conflict from your friend's story and adapt it to fit your story's setting and characters. You can also draw inspiration from the characters' traits or relationships and incorporate them in a way that enriches your story without feeling forced.
First, analyze the key points and emotions in your friend's story. See how they can fit into your plot or character development. Maybe use it as a subplot or a motivation for your characters. But make sure it doesn't overshadow your main story.