Charles Helliwell's Answer:
Managing or supervising a friend is a challenge we all face from time-to-time, Sally, and is rarely done well, to the benefit both parties.
First of all, don't be too hard on yourself, as often the first and most natural reaction is one which is drawn from the emotions of the friendship.
Remember that in work as in life, there is personal; and there is professional. Consequently, your first task is to separate the two.
Behaving as a professional and responsible manager is little different from behaving as a professional and responsible individual. In fact, you may well have earned your position on your ability to separate the two; whereas your friend may be stuck where she is because she cannot.
Envy and resentment resulting in disruptive and dysfunctional behaviour is a common enough outcome, although what is less understood is that the envy and resentment mostly emanates from a lack of confidence and self-esteem.
You can overcome this by engaging positively and directly with your friend and talking to her about how much you value and how much you've learned from her key skills and qualities. By making her an integral part of your success, you will be impressing on her the importance of her role as both a friend and a colleague.
But beware of allowing this relationship to sour your ability to manage your responsibilities and make the difficult choices you will face; otherwise you may become tainted with the dreaded brush of favouritism.
It is a very fine line that you will have to walk and a very delicate balance indeed. It is one which requires much practice to perfect and is one that so many managers fail to.
There is no easy way to ask a friend to do something they may dislike, but you can sugarcoat the pill by engaging with them first and asking for their opinion or viewpoint on how best to proceed. Sounding out colleagues is a common enough practice for most managers; just as long as you remember that you and only you, must make the final decision.
And if all this fails to engage your friend in a positive and mature manner ? Then the personal side of friendship must be brushed aside for the sake of your professional integrity. The name of the game in managing any group of individuals is to raise them to a higher level than they thought themselves capable of; not lower yourself to the standards they decide to set.