The series in general is this, as its premise basically runs on The Power of Friendship and shows how far a little bit of kindness goes.
Part I
Hermione's parents' thoughts on their daughter when she comes home for Christmas, when they think about how happy Hermione seems with her friends and how much time she spends talking about them.
There are several moments in this work in particular that show Harry, Ron and Hermione acting like a bunch of regular preteen friends, laughing about silly things and playing Exploding Snap.
Part II
Part III
Even though he was on the run, and pretending to be dead, Sirius hasn't forgotten about Lupin, speaking up for him passionately when Umbridge pushes her anti-werewolf legislation through the Ministry and later sending him some Wolfsbane as a Christmas gift.
