In this case it would hurt her though.
One: Mia's not stupid, she can figure out easily that it was her in the fic.
Two: You already apologised to her and said that you just wanted to put her with Kellin.
Three: You don't even tell a "small white lie" to your best friends. Because you should be close enough to them not to lie ever.
Author's response
I fixed the problem okay. I have gone through every chapter and changed Mia's name to Destiny, an OC I have made up within ten seconds, and I got rid of her nickname and called Destiny 'Des'.
I have changed the appearance too. Don't get me wrong, Mia will still be in this fic, but as like my adopted sister or something, or one of the kids from another school who I meet at the party that'll be the big plot twist. So, I hope that's kinda fixed the problem just that tiny bit..