How do I make him believe me?

My boyfriend invited me to the mall for tomorrow and with a few our friends..well the problem is he also invited a friend of mine cause he thinks we are "best friends" and I also tought she was my best friend...until a few minutes ago she told me a lie telling me that he doesnt me to go and that 2 of our female friends were gonna flirt with him and steal him away from me which it wasnt even true,I already asked them and theyre not even that type! So i told her and she got pissed and started telling me that shes gonna start flirting with him and because she likes him. Truth is..its not that she likes him,she likes every single taken guy there is,she did the exact same thing to my ex boyfriend but I gave her one last chance. But now enough is enough,im getting sick of her. Shes telling me all these lies,shes trying to steal him away from me and making my life miserable! What a "friend"! And so i told my boyfriend about it and after i told him everything..i asked him if he really wanted her to go with us and he said "Yeah i dont think shes gonna start flirting with me dont worry and if she does i wont let her because i love u not her". I dont want her to go...either she doesnt go or I dont go. But i dont want him to think im a ***** or i dont wanna argue,I really love him. So what can i do to make him believe me that shes no good girl,shes the worst friend you can ever have...I regret meeting her.