I dealt with this exact thing in jr. high and high school. I had this friend that talked garbage about every new person that came along and insisted that I not be friends with them. Eventually I gave in, every single time, and always this girl would turn around and become friends with the very person she insisted was not worth being friends with.

And now I'm the one who has no friends at all.

Talk to your friends and find a way to tell them that it's ok if they don't like Heather, but you do and you have chosen to be her friend and you would appreciate it if they would respect your decisions and treat your other friends as they want to be treated.