I would argue that your “best friend” should be accepting of all your decisions and should never be in the position of hatred in the first place. Sure they are free to not like them, and perhaps even hate them, but not at the expense of others. I have two best friends, and we agreed early on that we would never let each others plans or opinions get in the way of our relationship decisions. Furthermore, we also agreed that if one was in trouble, they could blame it on the other. For example I was blamed for renting a Playboy tape once. Of course it was my buddy, who did it, and his girlfriend was angry – but I gladly took the fall and he was in the clear. I did this for two reasons. Once is because he is my friend and that is what I’m there for. The other, I’ll take the fall all day long – I didn’t have to live with her :o
So in short, if your “best friend” hates your relationship partner and this hate is causing issues, then I would unfortunately suggest they are not a best friend, because they clearly hold their own feelings over yours.