My best friend is changing?

i hate to say it but my best friend of 4 years,anna, shes turning into a s**t and were only 13! i love her and everything but she needs to stop! anna met this new girl, lexi, who is really bad news. lexi is 13 also and shes already drinking and doing really dirty things with guys and its rubbing off on anna. anna is doing stuff with guys that is WAY too young for our age! shes suddenly soaking up attention from guys because shes acting like lexi. her and lexi had a sleepover and took some "" photos and uploaded them on facebook so all these boys can see. and they uploaded a video of themselves wearing bombshell type push up bras and short tight shorts, dancing really inappropriately. then they finally put on shirts but were stripping them off! and maybe they were just having fun but idk. anyway, i just want my friend back! seems like last week ago she was my best friend and now she made a huge turn around! and im not jealous at all of her, i understand completely that she is allowed to make new friends and people do change. but im worried for her when she gets older. if shes doing this at 13 what will she do when she gets older! what can i do about this? i just want my old friend back :( im really upset..its like i dont even know her anymore