What should I do about her?

Ok so this girl and I got together before school let out. I used to sneak over to her house and everything cuz she lived across the street. But we also had trouble being together cuz our parents didn't approve and this other girl was jealous of her. Later on she broke up with me cuz she didnt have the time for me and i was fine with it so we just became friends. So as i got bored of being single I looked for another girlfriend. Then my ex blew up on me because it was one of her friends ( and in the town i live in everyone is apparently your friend by some kind of means.) So when she got mad at me for flirting with another girl she asked me if i was serious about all the stuff I said to this other girl and suddenly she said she never wanted to see me ever again. This happened about a month ago and I'm worried about her because I think after that she's becoming more of a... well... not a good girl. I heard she's making out with this guy and that guy. I'm worried because her older sister goes around and does "it" with all her boyfriends and I'm afraid that my ex will. This isn't because I care for her but it's cuz I still consider her as a friend and I cant let any of my friends do that. I'm just wondering what I should do... whether it be that she goes and screws her life up or help her as a friend. Sorry the message was so long and if you read it all the way thanks.