In A Thousand Splendid Suns, how does the deep friendship between Mariam and Laila help them overcome their situation?

In A Thousand Splendid Suns, Mariam's and Laila's friendship inspires them to fight back against a very challenging situation.

Mariam and Laila suffer mightily under the oppression in their daily lives.  Under the Taliban, submission and silence are common reactions to such overwhelming difficulty. People suffer in silence because they do not believe their situation is going to change.

However, the friendship between Mariam and Laila is transformative.  Both women recognize the possibility of restorative hope because of their friendship. We see the transformative power of this friendship at different points. For example, Mariam is profoundly impacted when Aziza, Laila's daughter, is born:

As soon as she was in Mariam's arms, Aziza's thumb shot into her mouth and she buried her face in Mariam's neck. […] Mariam had never before been wanted like this. Love had never been declared to her so guilelessly, so unreserved.

Aziza's love strengthens the bond between Mariam and Laila. This bond helps Mariam recognize that she is worthy of love.  The affection she shares with Aziza and her mother is rooted in respect and dignity, something that both women use to challenge their situation.  When Hosseini writes how Mariam experiences the power of "unreserved" love, it shows how their friendship is capable of replacing silence with defiant action.   

In A Thousand Splendid Suns, empowerment results from friendship and love. Mariam, who had been seen as an outsider, "a weed," experiences a connection with Laila. As a result, she recognizes that she can be defined by more than her difficult situation:

Laila crawled to her and again put her head on Mariam's lap. She remembered all the afternoons they'd spent together, braiding each other's hair, Mariam listening patiently to her random thought and ordinary stories with an air of gratitude, with the expression of a person to whom a unique and coveted privilege had been extended. 

Love gives Mariam the power to see more than just suffering in her world. She is able to display strength that can allow her to challenge the evil that surrounds her.  

Mariam realizes that she is not alone because of her friendship with Laila.  She cannot acquiesce to silence because someone else depends on her and she depends on someone else.  Mariam recognizes that "Laila and Aziza—aharami like herself, as it turned out—had become extensions of her and now, without them, the life Mariam had tolerated for so long suddenly seemed intolerable."

The friendship between Mariam and Laila inspires both to take action.  Mariam kills Rasheed to save Laila.  She does not regret her actions. She knows that she acted in defense of her friendship with Laila.  At the end of her life, Mariam believes that "she was leaving the world as a woman who had loved and been loved back."  The transformation from seeing herself as an outsider who is incapable of receiving love to one worthy of it was only possible because of her friendship.  For her part, Laila escapes and will never forget to honor her friend through her actions towards other women and in the naming of her child, should she have a girl.  Friendship provides each woman with the hope that they deserve better and can insist upon it from the world around them. This galvanizes them into action, giving them strength to endure and eventually triumph over their difficult situation.