What should I do to get rid of my only and closest friend? I have a BFF and she and I are practically sisters. I love her but I learned it is sinning to be her friend as a muslim. What should i do?

Wow, it's a sin to be someone's friend? To treat someone with respect? To care about someone? To help them and allow them to help you? 

That's what friends do, and it should be regardless of their religion or lack thereof, good people are good people and if it's a sin for you to care about a good person, I guess I'd have some thinking to do.