Harry Potter is a series that is about so much more than just magic (although let's face it, the magic is pretty spectacular). Beyond being about a boy fighting an evil wizard, it's about growing up, learning what really matters, fighting different kinds of evil (Umbridge being her own special, all too real, type of dark witch), and of course, friendship.
It's the friendship between Harry, Ron, and Hermione that will always be front and center in the series, but there are so many more layers to the lessons about friendship in this franchise. The connection between the Marauders, between old friends and new ones, and in the lack of friendship between Harry and Draco are all a major part of what Harry Potter taught readers about how to be a good friend.
Stand Up To Your Friends, Sometimes
One of the best-known quotes on friendship in the series comes at the end of the first movie, when Dumbledore awards the winning house points to Neville for his attempts to keep the Golden Trio from sneaking out. "It takes a great deal of bravery to stand up to our enemies, but just as much to stand up to our friends.' Wise words, as usual, and this is far from the only time that this crops up in the franchise. Whether it is Hermione getting sick of Ron and Harry not speaking, Ron reminding Hermione she is a witch, or someone else entirely, the series clearly makes the point that it's ok to call friends out when they deserve it.
If You Don't Have Anything Nice To Say...
Admittedly, if Ron hadn't been loudly talking about why 'nobody liked' Hermione in her hearing, the three might never have become the friends that they did (and without Hermione, Harry and Ron wouldn't have lasted through the first book!). However, it's still a reminder that it's often best to just keep those meaner thoughts quiet - as Ron's words left Hermione crying in the bathroom, and nearly killed by a Mountain Troll!
Forgive Your Friends, When They Mess Up
The friendships in Harry Potter are heartwarming, but far from perfect - and that leads to plenty of opportunities to teach about the importance of forgiveness. Obviously, one of the biggest moments is when Hermione first forgives Ron and Harry for being mean about her - and when they forgive her for being a know-it-all!
This is seen in almost every book, though, and even on the search for Horcruxes, Ron loses his temper and storms off, leaving Hermione and Harry alone. However, when he does come back, they forgive him... even if it takes Hermione a little longer than Harry!
Choose Your Friends Wisely
When Harry first meets Draco, the young Malfoy attempts to befriend him (and what a different series that would have made!). However, Harry has noticed how Draco sneers at other children, and how welcoming Ron was in contrast, and makes a powerful decision about choosing friends who are kind - although perhaps, had he and Draco been able to connect, they wouldn't have spent so much time trying to kill each other! Harry's parents, too, learned the hardest possible lesson about choosing friends, when they put their trust in Peter Pettigrew... and lost their lives for it.
Tell Your Friends When You Need Help
One of Harry's biggest flaws was his tendency to avoid asking for help, even when he needed it - and the times he struggled most was usually when he was trying to deal with everything alone. When he reached out to Ron and Hermione for help, they could do pretty much anything... but when he ignored them (like when trying to work out the Egg clue for the Triwizard Tournament), he almost always ran into trouble.
Celebrate Your Unique Strengths
While Harry, Ron, and Hermione are best friends, they are also very different - with many theorizing that they each represent one of the other Hogwarts houses (Ron as Hufflepuff, Hermione as Ravenclaw, and Harry as Slytherin). This isn't a bad thing, though, and they actually work best together because of this. As shown as early as the first film, when they have to work their way through various magical tests to the Philosopher's Stone, they each have complementary strengths - and they are quick to celebrate each other for that.
Stand Up For Your Friends (Even When They Aren't Around)
Each of the Golden Trio is quick to stand up for their friends, no matter what. They defend each other at every turn - even when they aren't around, or it wouldn't get back to them.
When Harry meets Professor Slughorn, he makes a comment about 'muggle born' students, and Harry lashes out immediately with the response that his friend is muggle born, and one of the best in the year.
Friendships Should Bring Out The Best In You
In part because of their differences, the Golden Trio balance each other out - and like any good friendship, bring out the best in each other. Hermione keeps her friends focused, and reminds them of the importance of learning, while they help to bring out her rebellious side - and that's just one example. Compare this to Draco, Crabbe, and Goyle - three 'friends' (or a bully and his henchmen), and it's easy to see how friends that bring out the worst in people create toxic dynamics.
Be Honest With Your Friends
Ron and Harry often keep secrets from each other - and it never really turns out well for them. Ron is ashamed of his jealousy, his feelings of inferiority, and his fears about Harry and Hermione (and his feelings about Hermione, come to that), and it all builds up to lead to them having two major falling outs. If they had just been a little more honest, with themselves and with each other, it would have been better for everyone.
Accept Friends For All That They Are
The best friendships in Harry Potter involve accepting people for all that they are - even things that may spark prejudice in others, or seem a little bit strange. Sirus and James were great friends to Remus, despite his being a werewolf, and all the fears that would usually create in the magical community. They even went so far as to learn to transform into animagi for him, so he wouldn't be alone on a full moon. Whether it's Harry's way of making Ron feel like his lack of wealth isn't a problem, or Hermione's way of letting Luna know that she cares about her (even if she doesn't believe in all the same things), Harry Potter is all about acceptance... even down to the moment in the finale where Harry and Draco nod to each other on the platform, accepting everything in the past, and moving forward.