Should I tell her parents?

Erin • In love with the most amazing man in the world 10.20.12 ❤️
I used to be very close with a friend but we are no longer friends I still care for her and don't wanna see her hurt or making bad decisions I know of at least 3 boys she has been sexually active with and I have seen her smoke weed she is only 15 and when I lost my virginity her she told her parents and they told mine in hopes that if they found out my no longer friend was having sex or making bad decisions that they would tell them I've told my mom mind you I'm 18 but she didn't seem to care but I don't know if I should tell her folks in hopes that they could help her I'm not trying to be a back stabbing bitch either let me know what I should do I just don't want to see her get hurt even though we're not friends anymore