Well, that's a little bit complicated and im not sure if i had understood the whole sitution. How did you learn to know each other, do you go to school together? And the other friend too?
And did she was sweet, friendly, nice etc. at start and now mean, liar, cheater etc. or is it a change between the days (so one day like this, one day like that..)?
Now to you: Did you try to talk with her in a normal way before you stopped talking to her? I think it's very important to try to smooth such a thing over (i hope that's the right expression xD ) while talking.
If you ingnore her without a caution, she doesn't understand the situation and the reaction rightly.
How does she behave? Does she fight for your friendship or does she ignore you, too?
So to your special question: it is right to do something, she can not behave like that in a friendship. But at first you should talk with her etc., after that you can omit to speak with her
