Did I do the right thing letting her go?

she went to a lockin at a theme park. When I called they said there is no adult supervision so basically they can leave if they want to. She said she would behave and so did her boyfriend. She is only 15, I think I should have stayed with no but so many of her friends were going and she was crying when I said no. Her b/f mom just picked them up and I told her my woes, she said she felt the same way and figures they will get bored soon so she will be picking them up but I am still worried. I know I am over protective most of the time and I am trying to give her some time with friends but I am still a mom right? I told her to call me every so often and not to leave that place and if her friends leave she is to stay there and call me. Did I do the right thing?