Before you are elevated to the position of Manager, let it be clearly understood that you do not give special preference to anyone because of personal relationships. The way you treat Mr. A should be the same way you treat Mr. B.
If you are promoted, your friends should already know your values and standards. Being fair and consistent is not being bossy or rigid — it is being disciplined and protecting your integrity.
Do not compromise your principles in any way, because once you do, your integrity will be undermined. Outside the office, you may relate freely and offer assistance where appropriate. However, in the workplace, always follow due process and uphold the ethics and standards of the organization without compromise.
When you lead with fairness and integrity, your true friends will understand — and they will remain your friends.