Bestfriend or boyfriend? I'm stuck in the middle :(

My Bestfriend and boyfriend hate each other. My boyfriend's just really misunderstood. She thinks he's a terrible person and she sent him this really hateful message and she refuses to apologise to him.
My Bestfriend doesn't live in the same state as me anymore. She moved a couple of months ago. She's visiting in October. Even my relationship with my boyfriend is long distance because he goes to boarding school in another state. He's coming back in October too. So I'm really confused who I should be spending time with
My friend sent him really terrible things. She refuses to apologise. I feel like I should stand by my boyfriend because he would definitely do the same for me. If one of his friends spoke to me this way , he would definitely cut that friend off. He told me that I can do what I want. He's ok with me still talking to my Bestfriend and everything but I know that he'll feel really bad if I spend time with her instead of him, when they both come back to the city in October.
She's my Bestfriend of 11 years. I love her to bits but I don't love my boyfriend any less. I just don't know what to do when they both come back home at the same time. Which end do I go? Please help :(

Before I can give any advice, what is their beef with each other??? I need to know what transpired between them because I find it odd that a friend of 11 years just "misunderstood" your BF for no good reason. Maybe it's something you don't care to acknowledge instead.

my hunch is you have complained about your bf a lot to this friend. Or she is jealous of the relationship. If its complaining, you should explain to her that its just complaining that there are so many goods that equal out the bad and you feel she is the best person to talk to. and maybe you can see both. if its jealousy, take a break from her for awhile and let her come back

This seems rather unprovoked, so I have to assume you arent telling one side of the story. Did you complain about him a lot to her?