Betrayal by friend or should I just let it go?

So long story short:

Three friends, me and my best friend, plus a friend of my best friend who after about a year or so I became quite good friends with too.

Turns out the 'friend' has been ****ging me off, really quite nastily, towards my best friend. All in some sort of blatant attempt to turn her against me.

Obviously, now 'friend' is not a friend and is a two faced you know what. Despite my best friend agreeing with her bitterness and actually admitting she's not a huge fan of her because of the way she's been to me, my best friend is STILL meeting up with her all the time. Behind my back.

She thinks I don't have access to social media because I'm not on Facebook, but some of my other friends get all her stuff on their feed, so I see it all. My best friend now lies about meeting up with her, despite the fact them meeting up is plastered all over Facebook.

Why is she lying about meeting up with someone that has REALLY hurt me? I'm not expecting her to stop ever seeing this girl (although it does make me question her loyalty, because if the ball was on the other foot I'd be fuming if someone was being downright vile) it's more lying about not seeing her........and still being all over her despite what she's said about me!

Neither of them are friends, i'd drop them