I would first sit her down and express your concern, I think kind and compassion is the way I would approach it always being aware tough love may have to come into play. I have managed very large groups of people in my careers, at the end of the day, friend or not if they are not doing their job while at work, its time for them to go and I never allowed work calls to me at home. Keep in mind your responsibility as a supervisor and you do not want her problems to become yours, always document any meeting for the HR file.
Andrew