Friendship is certainly a dominant theme in Walter Dean Myers' Fallen Angels, a novel about youths fighting in the Vietnam War. In fact, the word friend can be found many times throughout the novel, especially in references to friendship. For example, later in the book, while in the hospital with Peewee, the novel's protagonist Perry asks to look up the record of his friend Judy Duncan and is told that his "friend didn't make it" because she was transferred to a field hospital, which was bombed (p. 307).
In addition, while Perry is waiting to board the plane heading back to the states, he watches the caskets be loaded first. His thoughts while watching the caskets load reflect on what it's like to come out to Vietnam and have to fight together and build friendships; yet, death is a personal and solitary experience, so each man dies alone:
The caskets were almost completely loaded into the tail end of the C-47. They were there together, but they had died alone. Maybe some of them had been friends. (p. 308)
Jenkins had been walking with me and talking with me only hours before. Seeing him lying there like that, his mouth and eyes open, had grabbed something inside my chest and twisted it hard./
This quote shows the importance of friendship and how hurt the protagonist is when he sees his friend's death.
This is the only significant one I found, I'm sorry if this wasn't helpful!