best friend?!

Okay so I've had this best friend since I was about 5, I'm going to the 21 soon. She has a boyfriend of 5 years, things are a little rocky between them. She's always been that goody goody Christian girl who went to clubs but never did anything bad. I left to the army and now all of a sudden she's talking to other guys constantly, wearing overly low cut shirts, trying to show as much cleavage as possible, always getting overly drunk with new friends, sending and posting indecent snapchats to me and her story, and I'm pretty sure she is cheating.. I don't want to loose my friend but she's becoming a person I don't know, what do I do? Nothing, talk to her? What.. 😔