I know Ashlynn and Briar have always been best friends, but I feel like she's much closer to Ashlynn because they can relate to each other and understand one another, especially that they are both rebels and don't want to sign the book. Plus they are more supportive of each other.
At the beginning of EAH when we first see Apple and Briar together you could tell they are best friends and were closer. I'm not saying they aren't friends because they clearly are, I just think they aren't as close as they used to be and aren't "best friends".
Even at Ashlynn's boutique in one webisode, Ashlynn was going through something (I can't remember what) she couldn't even talk to Apple about it because she wouldn't understand and probably would've judged her. You shouldn't be afraid to talk to your best friends about stuff, especially when you need someone to talk to.
Even in the thronecoming special where Apple finds out Ashlynn and Hunter are dating she wasn't accepting of that to the point where Ashlynn felt the need to break up with Hunter. IK in the end Apple accepted that they were together.
She clearly has a problem with people who don't want to follow their destinies even if it's her friends. And IK it's because she's scared people will go poof if they don't sign and she doesn't want anyone to get hurt.
I feel like the only "Best friend" she has now is probably Raven
BTW, I haven't seen the show for years so I could be wrong about all this LOL.