am i being unreasonable

i just finished my sophomore year of high school, going into my junior year. i'm almost 17. i'm crazy involved in my church & extremely close with my youth group. there's one girl in it who i used to be best friends with for years up until recently. to sum up, she went from being positive, nice, & all about God, to sneaking guys (and girls) over to have sex w when her parents weren't home, drinking, and doing drugs. i'm absolutely done. i refuse to associate myself with people who make poor choices like this, as i'm a firm believer that your friends and who you surround yourself with heavily influence you, and i can't deal with that. it's also not even her choices, she makes you feel so bad about yourself and is so judgmental. she has no consideration for others. i just don't get how someone can change that drastically in a matter of 8 months. 
so my question is, is me completely wanting to dissociate myself with her being unreasonable? i just don't know what to do anymore. all i know is i really don't like how she treats others OR her choices. ugh i need help. 
UPDATE: to give more info, the reason why i'm questioning if i'm being reasonable or not is because i talked to my mom about it and she said maybe it's my job to be the positive influence in her life, but i really don't want to BE in her life.