It's not about tattling on your coworker. It's about patient safety. Policies are there for a reason, to protect the patient and provide the best care. Fill out an incident report. It may be that that nurse or other nurses just need a review of the policy. Incident reports are not meant to be punitive, but to help recognize where education may be necessary to prevent errors or improve patient care. Never feel bad about doing what is best for the patients. Luckily for that patient there was no harm, but the outcome could have been different...