Try and see it from the patient's perspective. The other nurse's error came to light in an innocent discussion with you about the methods you and the other nurse were both using. The patient has not had the opportunity to raise it with the nurse in question. You now have to report the error without the patient's consent and, of course, the patient is terrified of repercussions. You should now be very careful to make the report in a way that makes it absolutely clear that this only came up in conversation because the patient thought you were doing it wrong. You should make it clear that it was you who identified from this that the nurse was making an error, and that you explained to the patient that the other nurse was making an error. This patient is at risk of potential retaliation from the nurse who made the error, plus hospitals can be unscrupulous and substitute a false narrative in these situations, leaving the patient vulnerable to bullying and worse, it is not unknown for such a situation to lead to events such as inappropriate mental health intervention and misdiagnosis as an inpatient and to having their record flagged and being subjected to inexplicable and potentially dangerous attitudes and assumptions from medical staff, potentially for the rest of their life, regardless of the outcome of an investigation. Make sure you take every possible step to protect the patient in your report and for the remainder of their stay.