Do I stay her friend or never talk to her again...?

[ineedadvice!]

Okay. So some background on this story. Me and this girl "Sarah" have been bestfriends since we were two years old. We dance together at a studio now for 14 years. A new girl Tara came to our studio this year and She sometimes lies and tells her mom she's sleeping at my house but she really sleeps at her boyfriends.

So here's the issue:
Other than me Sarah and Tara there is three other girls on the senior team at dance. Sarah does a lot of stuff to annoy us and she tries to turn us all against each other. Sarah picked a basketball game over coming to dance, a week before competition so we were all pissed. We texted her in a group message saying basically saying how she never comes anymore (she never does) and how we don't want to look bad on stage. Anyway, she basically flipped out on us and said a lot of rude things and actually messages taras mom on Facebook saying how she's been lying and sleeping at her boyfriends. She screwed Tara over and said rude things to us.What do I do?