Is your daughter into music in a big way? Just that there's the Creamfields festival on near Runcorn/Liverpool, so not a million miles from Salford if that might be somewhere they could have gone.
Hope she's in touch soon, and while can understand concern, hope you can try to stay calm and not explode when she returns, as she's old enough to say "I'll do what I want" and flounce out again...
Know you're worried, angry, and unhappy {with her and her dad}, but even though she knows you are concerned (when she bothers to listen to voicemail, sees text messages) and will know she'll be in trouble when she gets home, she might react badly and overstep even further if she thinks she has nothing to lose, hence my concern that having a big row about it could be worst possible thing...
I'm sure she knows (deep down) you are concerned because you love her, and concerned for her safety, and consider her friend least suitable as a friend, but explaining you're cross because you care rather than mention the friend, might help her see you're not being a killjoy, but have concern about her safety if she says one thing and does another.
(Only know about Creamfields as I heard it mentioned on R1 on Saturday while flicking across the stations while I was on a train - never been much of a festivals person myself)