To tell or not to tell..

A staff nurse in a pediatric surgical unit had miscalculated a child's insulin dose and had given her to much medication.The child was found in a hypoglycemic state.The 6 year old child diabetic patient was in hospital recovering from minor surgery.She was found looking very pale and was perspiring.Staff nurse B found the child in this hypoglycemic state and called on her BEST FRIEND staff nurse A and together they did a blood sugar test..The child was transferred to intensive care ,recovered and was sent home a week later.
Staff nurse B stated how surprised she was to find the child in this state .Nurse A finally admittedthat it was her fault and had miscalculated the child's insulin dose and had given her too much insulin.Staff nurse B asked if nurse A had filled out and incident report and Nurse A responded by stating she did not intend to report it as it would create an inquiry and trouble for her.She could do with out this right now as she was being considered for an advancement. and feared the incident would reflect unfavorably on her employment record also stating to nurse B (Best friend) "and I hope your not going to report it, either."I told you this in confidence and as a friend, and it would be unethical for you to do anything about it"
What should nurse B do???