I was recently promoted to manager at my company after working here for 10 years and being a peer to the people who are now my employees. Needless to say, the transition has been awkward. I've tried my best to remain friendly yet professional, and I've stopped going to the lunches and after work parties like I used to. Reason being, I'm trying to make it clear that I am now the boss so we can't be pals like we used to be. Most of them get it, but not all. In fact, one gal who I used to lunch with often has been coming in late a lot lately and leaving early. The first couple of times she had decent excuses so I didn't think it was a big deal. Now I'm beginning to think she's taking advantage of our former friendship.
Learning As I Go
Just because you're the manager now doesn't mean you need to blow off friendships that you spent 10 years building. Yes you will have to separate work and friendship, and make a point to not play favorites, but there's nothing wrong with you remaining friendly with your friends! Go to lunch with the group on a rare occasion and definitely attend a staff after work party once in a while. You're a manager now but that doesn't mean your social life has to end. As for the slacker pal, call her into your office and have a "Come To Jesus" as my mom would say. Let her know that she has to be on time to work, and leave when the day is officially over, or she'll be written up. Then explain the company policy on the number of warnings it takes before an employee is fired. Afterwards, say something like "on another note, how are things at home?" or something to that effect to let her know that you still care about her as a person even though you need to kick her butt as a boss.
Best of luck!