Is this girl really a friend? Should I still be her friend? HELP!!!?

So I have a friend that has been my friend since 6th grade til now, we're 19 now. This OTHER girl let's call her Ria, I met last year and she was really nice at first to me and then turned into a total.. you know the word. She had countless arguments with me and talked a lot of crap about me behind my back and to my face. I finally decided after 2 years of this we need to end this friendship so she deleted me off of Facebook then I blocked her, and now we're not friends anymore. NOW... my best friend is all of a sudden good friends with Ria. While I was in Seattle, they started hanging out a lot and now they're great friends. My friend knows all of the stuff that is going on between Ria and I from RIA! So basically, she's hanging out with her and talking bad about me. Recently, my best friend told me she will never hang out with Ria behind my back and tell someone not to tell me. But yesterday, I found out she did just that from my other friend who made me swear not to tell anyone. So basically, my friend since 6th grade lied to me, and has talked a lot of smack on me with my almost enemy, Ria. Lol today she called me to hang out with her because we were planning to today, but after I found out she did that behind my back I acted kind of to the point on the phone and said I was busy. She could tell something was up so should I still be friends with the best friend? Or drop her just as I dropped Ria? should I apologize for being rude on the phone? Sorry if this was confusing! But please answer, it'll help me :) Thank you!