The narrative of To Kill a Mockingbird never specifies whether Calpurnia and Tom Robinson are friends, but it can be assumed that they know each other since they both attend the First Purchase Church in the Quarters. This fact is revealed in Chapter 12 when Jem and Scout accompany Cal to the church and Reverend Sykes announces that the collection will be given to Tom's wife, Helen. When the pastor determines that the collection is not big enough, he chastizes the congregation and asks for more. Following Tom's death, Atticus requests that Cal join him when he has to break the bad news to Helen.
Calpurnia knows Tom Robinson well enough to give Atticus some background information on his family. In Chapter 9, Atticus explains to Scout that Calpurnia told him, Tom Robinson was a member of her church who she considered "clean-living folks." One can assume that the African American community of Maycomb was relatively small and close-knit. In a segregated society, African Americans relied on one another, worshiped together, and were familiar with everybody in their community. In Chapter 12, Calpurnia is able to explain Helen Robinson's situation to Scout which supports the idea that Cal is a friend of the Robinson family. In Chapter 24, Atticus interrupts Alexandra's missionary circle to ask Calpurnia to travel with him to Helen Robinson's home to give her the news of Tom's death. In Chapter 25, Scout recalls the story that Dill told her about Atticus and Cal's visit. He said that after Atticus left Helen's home, Calpurnia stayed behind. The fact that Cal stays with Helen suggests that she was a close friend of the family.
Tom Robinson needed Atticus to defend him, otherwise he knew he wouldn't stand a chance. This was a different era, and it was truly heroic that Atticus chose to do such a courageous thing.
Atticus is to defend Tom Robinson.