As a woman who has a male best friend, I will tell you that this girl was out of line. She was being superficially friendly towards you in order to look good in front of your boyfriend. Truthfully, it sounds like she has a crush on him and is trying to make you feel insecure so you will leave him. If they have been friends for a long time, then it is very unlikely that he will take your side over her's. If you really like this guy, then here is what you say to him in private:
"I think your friend seems like a lovely person, and I'm glad that you have a good friend in your life. However, I found it to be inappropriate of her to talk about people thinking you two slept together. This made me feel very uncomfortable. The fact that she speaks to you for hours over the phone and glared at us when we were kissing is making me think that she has a crush on you. Again, she seems to be a great person overall, but it would make me really happy if you would set some boundaries with her."
He will probably deny that there is anything going on between them. Then you should say "There is nothing going on at your end, but she is sending very clear signals that she likes you. Normally, when a female friend meets the girlfriend, she tries to make the girlfriend feels as comfortable as possible about the friendship. She strikes me as an intelligent person, so I'm sure she is completely aware that talking about how people think that you both slept together is only going to make me uncomfortable about your longterm friendship with her."
If he says you are being unreasonable or crazy, then say "Look. I'm not saying you need to end the friendship with her. All I'm asking you to do is set some boundaries. Would you be comfortable if I talked for hours with a male friend after 9pm? Would you be comfortable if he sent me long text messages every day or talked about how people think we slept together? You would probably want me to set some boundaries, right? Most men in that scenario would want their girlfriend to end the relationship with her male friend. I'm not even asking you to go that far. I'm just asking that you and your friend be respectful of our relationship and set some boundaries. I care a lot about you and I hope you care enough about me to set some boundaries with your friend."
Make sure you have an idea about what boundaries you want your boyfriend to set with her before you have this conversation. Be very specific and direct about what you want. Do not expect him to read your mind. If you don't feel comfortable with them hanging out together (without you) after 5pm on the weekend, then say so. Think about what you feel comfortable with. Of course, if you want him to stop being friends with her altogether, then you better dump him immediately because he is not going to pick you over her. Asking a man to choose between you and a close friend is not going to work in your favor.
If he doesn't want to set boundaries and still calls you crazy, then just say "If you don't want to respect me, then I think it's time that we break up. I wish you good luck in future relationships. You will definitely need it because no self-respecting woman will tolerate this type of behavior. " Then immediately walk away.
While it's fine to have friends of the opposite sex, it is crucial to set boundaries when the friend is in a relationship. I can tell you that I refuse to hang out with my male best friend after 7pm. I do not call or text after 5pm and I never speak more than 15 minutes to him. Yes, it sucks to not hang out as often, but out of respect for his relationship with his girlfriend, I keep my distance. I never want his relationship to fail because of me. This is how your boyfriend's female friend should have approached things.