In my opinion, the theme of friendship is prominent in Solzhenitsyn's novel One Day in the Life of Ivan Denisovich. In fact, Solzhenitsyn starts with this theme right away. Desperate at the beginning, Ivan depends on his memories of friendship in order to learn the skills of survival. Where he would have found despair, now Ivan finds the will "to get up" further adding that he "never overslept."
A second example has to do with the character of Kurzyomin. Kurzyomin was the first to establish friendship with Ivan as soon as he arrived at the concentration camp. How did Kurzyomin do this? Simple: by telling stories around the campfire and giving Ivan words of advice. Kurzyomin has lived at the concentration camp a very long time. As a result, he knows the drill. Put simply, Kurzyomin knows how to stay alive and wants to share that "how" with his new friend. What was this important advice shared around the campfire? To follow the rules and live at attention. Make survival your ultimate goal.
A man can live here, just like anywhere else. Know who croaks first? The guy who licks out bowls, put his faith in the sick bay, or squeals to the godfather.
There is also a more general theme of friendship from prisoner to prisoner at the camp. Sometimes these prisoners are named and sometimes they are not. Because they are all confined to the camp and all in the same prison, they are all in the same "club" and are, therefore, "friends." For example, when Shukhov gets punished for not awakening on time, the narrator explains that Shukhov's friends would have stuck up for him and perhaps lessened his punishment which, for that infraction, was being sent to the hole. Unfortunately, this good will would have done nothing for Shukhov. It would only have served to get others in trouble; therefore, the group of friends had to stay silent.
[They] saw Shukhov being led out, but nobody said a word: what good would it do, whatever you said? The foreman might have put in a word for him, but he wasn't there.
Or how about this bit of wisdom from another fellow prisoner:
You should rejoice that you're in prison. Here you have time to think about your soul.
In other words, it is more important to worry about your afterlife than it is to worry about the discomforts and the evils of this life on earth.