midcomp-- very hard for me to help , do not have children of that age . But as a parent you have to stick by what you say. Take away the phone and do not return , cut the internet link forever.She will still lie and try to meet these people -- so keep an eye on where she is going -- a disco take her , pick her up and so on.
You do not have to explain why to a 14 year old , just tell her the rules and do it.
She will not like it , but matter not , she will be as safe as you can make her .
She may even thank you in the future when her friends are up the duff so to speak and she isn't.
It is not going to be easy for any of you -- expect tears , tantrums and maybe even running away to a friend-- but you are the adult here what you say goes.
All the very best .Brenda.