My best friend lied to me, what do i do?

to start off we're both 14. but shes a freshman and i'm a sopre. shes 8 days younger. so about two days ago i said to her.. "if you ever smoke, please tell me. because i definitely wont be mad unless you don't tell me." and she was like "don't worry, i'll make sure to tell you." and then the next day i see on her twitter that she was having cigarettes and ice tea with her boyfriend.(who is a senior, juvenile delinquent, smoker since 9 yrs old. but thats besides the point. i just don't like him.) but.. it was the day before i told her that if she ever smoked i wanted her to tell me. so i ask her and she was like "i was drinking ice tea, my boyfriend was smoking." and i was like.. "ohh okay!" and so later i see her formspring, and someone had asked if she smokes cigarettes and she said "yes." and then people were like "i'm so disappointed." and she was like "its not as bad for you as you think." so i know for sure she smokes and i care about her a lot. she knows everything about me and i thought i knew everything about her.. i never gave her a reason not to tell me so i don't know why she wouldn't.. i don't know what to do about this.. like, how do i talk to her about it? she thinks its cly and that it looks good and that its not bad for you. i'm not mad that she smokes but i'm really worried about her.