Have you had a conversation telling her about the behavior before now? I’m assuming she’s been flirting with him before now. I mean you’re friends you know her well. Is she doing this and not knowing? I say that because we know the nature of our friends and what they have done when most don’t.
If she’s your friend a simple conversation to her first is more than appropriate. Are you willing to lose a friendship over non communication? You can speak to her and hear her out. After you speak to her than move forward in whatever way the conversation leads. It’s one thing to have random women flirt with your man its whole different story to have a close friend do it.
The next thing is to have a conversation with your man. Let’s be real as long as the both of you are on the earth attraction from and to you and your partner will occur. The one thing to always check is your insecurities and make a plan for how much is too much. If you have been keeping up with ToiTime I’m always encouraging couples to develop their own plan for things like this.
My hope is that you and your friend can move forward. Let her know that you’re uncomfortable with her behavior around your boyfriend. Hopefully she is open to receive from you. Good luck!