There’s no power stronger than friendship. As long as you believe in yourself and your friends, you can become the master of anything. Sure, some smarts, training, and skill can help, but feel free to neglect those if you’ve got some friends at your side. And of course, there’s always the villain there to mock your belief in your friends, but really he’s just sad because he doesn’t have any of his own friends. But don’t worry — you’ll teach him a lesson he won’t forget (and maybe even become his friend afterward).
Why this can be bad: While having strong, healthy platonic relationships is important to everyone’s well-being, it shouldn’t necessarily have the power to increase a monster’s attack strength or beef up your own power so you can take down a biomechanical super-demon. And should it have those effects, many stories leave out the true why for that by simply explaining it away as “It’s because they’re such good friends!” or “The power of their friendship did it!” which ends up sounding more like a deus ex machina than an actual explanation. And, honestly, it usually is. This excuse can be used to explain why characters can predict the future, escape mind-control, and survive days without more than a bite of food or a drink of water. Characters can essentially use this to worm their ways out of essentially any problem, continuity and sense be damned. Therein lies the biggest issue with this trope: the power of friendship doesn’t have limits or laws, and audiences either reluctantly buy it or completely disbelieve.
How you can fix it: Like everything, there needs to be boundaries. If your characters are solving ancient, unsolved mysteries simply because of their friends or are suddenly more powerful because of them, there needs to be an actual reason other than “friendship.” Now, if you’re actually going to set up rules that allow for friendship to have magical properties, great! That’s an explanation, and as long as you put rules and limitations on it like any other magic system, you should be good to go. But if you don’t, you run the risk of turning this into a cheat to get characters out of tough situations, and your audience will be left wanting. There’s no tension in a story if the audience can say, “I don’t know how they’ll get out of this, but it’ll probably be some friendship BS.”
Bottom Line: At a personal level, friendship is a powerful driving force, but if it’s somehow influencing fate or skill, maybe think about how that’s actually happening.