The theme of friendship, particularly the theme of betrayal within friendship, is especially expressed in Hermia and Helena's friendship.
We repeatedly see just how close Hermia and Helena were as children. In fact, the woods in which most of the play is set actually served as a place where they spent a great deal of time together as children, pouring out their hearts to each other. In contrast, now the woods is serving as the setting in which their friendship is broken apart. We first learn that the woods is a symbol of Hermia and Helena's childhood friendship in the opening scene when Hermia discloses her and Lysander's plan to escape Athens, as we see in her lines:
And in the wood, where often you and I
Upon faint primrose-beds were wont to lie,
Emptying our bosoms of their counsel sweet,
There my Lysander and myself shall meet. (I.i.219-222)
This passage not only portrays the theme of friendship by describing how close Hermia and Helena were as friends, it is also sets up the woods as the ironic setting of betrayal.
It is Helena who makes the decision to betray her childhood best friend Hermia by informing Demetrius of her and Lysander's plans to escape Athens via the woods, as we see in Helena's line, "I will go tell him of fair Hermia's flight" (251). Since the line portrays betrayal, it not only depicts the theme of friendship, but it also portrays the theme of friendship betrayal.
Ironically, even though Helena betrays Hermia, she later believes that Hermia has actually betrayed her by joining with the men in mocking her, as we see in her lines:
Lo, she is one of this confederacy!
Now I perceive they have conjoin'd all three
To fashion this false sport, in spite of me. (III.ii.195-197)
Helena's accusation of Hermia's betrayal in the very same woods they bonded in as children, shows us that the woods is being used as an ironic setting to portray both friendship and the betrayal of friendship. In this same speech we see further references to the women's closeness as children when Helena reminds Hermia of how they used to do and share everything together and asks Hermia if she will now break up their friendship by joining with the men to mock Helena just for the fun of it, as we see in her lines, "And will you rent our ancient love asunder, / To join with men in scorning your poor friend?" (218-219).
Therefore, these references not only portray friendship as a theme in A Midsummer Night's Dream, they further serve to portray the betrayal of friendship as a theme.