I resent my friend's flirting
What is wrong with how she flirts?
She was flirting with my boyfriend.
I can understand how you are upset. It's okay to be upset with your friend if they are doing something that annoys you. I think you should talk to her about boundaries and make sure your boyfriend isn't encouraging her as well.
She needs to stop doing this now.
Then tell her. Tell her her flirting with your boyfriend is annoying and uncomfortable with you.