|these girls love the sun|
I see best friends in YA all the time. There are good BFFS, a la Jellicoe Road and horrible BFFs a la Before I Fall. There are heartbroken friends and mean friends and chirpy friends and quirky friends and happy friends and every single kind of friend under the sun. Best friends, if they're used wisely and have a role in the story, can enrich the plot. If they aren't used wisely they can become nothing more than a trope or a plot device.
Below is a situation I see often in YA. This is not necissarily *every* best friend situation there is.
Cool MC: Hey, I'm super cool. I have a BFF. She is cool too (names reason BFF is cool). We hang out all the time.
MC: Oh look, a mystery/hot boy/problem /every single thing under the sun we need to figure out.
BFF: I'll help
MC: Wait, whoa! MY BFF IS NOW ACTING WEIRD.
BFF *acts weird*
|I hate drfiting away BFFs!|
I see this ALL THE TIME. It's predictable and annoying. And it's a sorry excuse for a writer. Don't just push your characters away like that. Plot device characters are annoying. Best friends seem to be the ones chosen for this most normally. Develop your best friends. Give them emotions. Feelings. Give them more than three scenes. Give them a real relationship with your MC. Make the relationship different and nonpredictable. Make the MC a character.
I don't feel any relationship towards your characters, towards your MCs, when you push them away and make them plot devices. Make your best friends strong, give them real purposes in your story. And if your MC's best friend exists only for one reason, to further the plot, to add suspense, then develop them. And if you have to, cut them apart with scissors. Make a best friend that someone would really be friends with.
Don't make your characters tropes. Don't make them plot devices. Because when you do, they become predictable. They become annoying. And they aren't best friends anymore.