The Witness by Nora Roberts

The Witness - At 16, Elizabeth Fitch has been dominated by her mother, doing everything her mother wants, even attended medical school.  However, she is sixteen, and has decided to dress and do what she wants.  She goes shopping and with the help of a new friend, Julie, buys all new clothes, then after producing fake IDs, she and Julie go to a club.  There they meet the owners and getting drunk, agree to go to their home.  While there, Elizabeth witnesses the execution of Julie's date and Julie.  She runs and is picked up by the police and put under FBI protection.  However, the killers find her and her protectors are murdered and Elizabeth goes into hiding.