What do I do if I feel jealous of my bfs best friend?
That depends. Why you feel jealous of her?
I was jealous because she has been hanging out with my boyfriend all the time
It's ok if you feel that way. Why not hang out with them more often so you can get to know his best friend and make you feel secure.
I have a better idea. I'm going to tell him that she's a bad person.
That is not a better idea. It is okay to be jealous, it is a normal emotion but you need to learn to manage it. You should talk to your boyfriend about how you feel.