My "friend" likes my boyfriend

Okay, so my "friend" Cassie, likes my boyfriend, Tyler. Well, Tyler FINALLY asked me out on 11/17/17 after school. He knows that Cassie likes him, and he doesn't really like her. He thinks that she's annoying and so do I. I want to stop hanging around her, but she's constantly following us around and trying to "flirt" with Tyler. I've tried to talk to her about backing off, but every time I try to have a serious conversation with her, she interrupts, and when I try to ask her to stop interrupting, she gets a really bad attitude with me! What should I do?