Call her into the office or somewhere private and give her a warning that she is acting inappropriately and that if she does it again she will get an official warning and be putting her job at risk.
Never correct, scold or rebuke a person in front of others; but this has GOT to be stopped the very next time that she does this. Point out that outside of work you are friends, but once she walks in that door *as an employee under you* she has to act appropriately too. Her behavior is jeopardizing both of your jobs and she needs to know this NOW.