Oh geez. That's such a hard situation. Trying to make your partner and best friend happy is almost always difficult. It's also usually more complicated than you think. I'm not sure if this is how your friend is feeling. But these are also usually underlying issues. She may be scared of losing you as a friend since you have a boyfriend now. A lot of friends feel threatened when a boyfriend enters the picture. If she's single, she may also be feeling like a third wheel and therefore jealous of your relationship. That doesn't mean she doesn't want to see you happy. It just means she wants that happiness for herself as well.
I would suggest talking to her nicely. I know you're scared that she'll get offended and it will lead to a fight. But this won't be resolved until you guys talk about it. But instead of just criticizing her for what she has said to her, make sure you reassure her as well. Let her know how much she and your friendship mean to you. And then let her know how it feels when she comes down so hard on your boyfriend. And that while you appreciate that she's trying to protect you, she also has to trust your judgment and give the guy a chance.
From there, I would just work on finding a balance. Spend some alone time with your friend, some with your guy and a bit with both. Maybe try spending time in a group setting when the three of you will be together. Just for now. Then your friend won't feel like such a third wheel. She'll gradually get used to him once she starts to see him as a good guy.
Hope this helps and she starts to relax more. :)
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