I think you already know she's not a friend. The other girls will catch on and leave her if you start the process by leaving first. They are probably afraid to stop being her friend because they were also threatened that they won't be able to talk to you (since you'd still be her friend)

Leave this mean girl in the dust and go make friends with Hannah (I have the feeling the "ugly" girl was way nicer and therefore a threat to the other girl)