One character that exhibits a lot of friendly love is Boo Radley. He tries to make friends with Scout and Jem as early as the first chapters of the book, and that love that he has for the children really comes through at the end after they are attacked. He starts his friendship by leaving trinkets and bobbles in the tree trunk for Jem and Scout to find as they walk home from school each day. One of these gifts is
"two small images careved in saop. One was the figure of a boy, the other wore a crude dress...they were almost perfect miniatues of two children."
Here, we see that Boo feels such a fondness for the children that he even has their images memorized in his head well enough to carve their figures into soap. He tries to extend a hand of friendship through all of these gifts. At the end, after he saves their lives, he is so worried about Jem that Scout takes him back to Jem's room where Boo gently reaches out and touches him to see if he's okay. This scene is a touching one where we see Boo's true friendly affection for the kids; he exhibits friendship love.
Other characters also show friendship love. Consider Dill and Jem; they are good friends; Scout and Dill also are friends, and show friendship love in their interactions. Cal and Atticus also are very good and trusted friends that have a loving, but purely friendly, relationship. All of these characters love each other as friends, and it is those ties that make the book so endearing. I hope that helps; good luck!