What passages in Harper Lee's To Kill a Mockingbird show life lessons about friendship and family?

In Harper Lee's To Kill a Mockingbird, the children's relationship with Arthur (Boo) Radley best represents lessons about friendship.

Arthur Radley is significantly stigmatized by Maycomb's society due to the fact he is a recluse who never leaves his home. As a result, Maycomb's citizens have circulated many rumors about why he never leaves his house, such as Miss Stephanie Crawford's rumor that he has been kept under house arrest by his father and now his brother for being mentally unstable. Due to the rumors and stigmatization, Maycomb's children have given Arthur Radley the name Boo Radley and frequently mock him.

Jem, Scout, and Dill particularly become guilty of mocking Arthur when Dill comes up with the idea to try and make Arthur come out of his house. One thing they do to mock him is re-enact Miss Stephanie's rumors in their yard, which is in Arthur's line of sight from his house.

However, despite mocking him, the children begin understanding that Arthur is doing kind things for them and reaching out to them in his own special way. For example, after a failed nighttime attempt to try and get a glimpse of Arthur in his window, Jem returns to the Radley property at 2 am to retrieve his lost trousers and finds them lying on the fence, neatly folded, and mended. The children also begin finding items in a knothole in one of the oak trees on the Radley lot and soon come to realize they are gifts from Arthur. The most noteworthy gifts are two bars of soap craftily carved to look just like the children. As soon as Jem becomes convinced the objects they are finding are gifts to them from Arthur, he sets out to leave Arthur a thank you note but is devastated to find that Arthur's care-taking brother, Nathan Radley, had filled in the knothole with cement. Jem is so devastated he cries because he feels guilty for having mocked Arthur and devastated by the fact they have no way to show Arthur kindness in return. We know Jem cries based on Scout's following narration the day Jem realized Nathan had filled in the knothole:

He stood there until nightfall, and I waited for him. When we went in the house I saw he had been crying; his face was dirty in the right places, but I thought it odd that I had not heard him. (Ch. 7)

Another memorable act of kindness Arthur shows the children is when he sneaks out of his house at night to cover Scout up with a blanket while Jem and Scout wait in front of the Radley property for the town to put out Miss Maudie's house fire. Finally, when Scout's and Jem's lives are threatened by Bob Ewell, Arthur is the one who comes to their rescue, risking public exposure by stabbing and killing Ewell in defense.

As Scout comes to realize by the end of the novel, Arthur acted kindly towards the children because he cared about them; he saw them as his friends, or, more specifically, as Scout phrases it, he saw them as "his children" (Ch. 31). Hence, Arthur Radley's actions in the face of the children's mockery gives us a lesson about the unconditional love felt by true friends.