Could a psychiatric nurse date a patient that is no longer in their care?

Ethically no: For a few reasons..."do no harm" first and foremost. There is a medical-ethical contract not to date patients, current or past. If it does not fare well, the patient's trust in and sense of safety with mental health professionals will be negatively effected, which could deter future treatment. As well, there always is a power differential that creates an inherent imbalance in the relationship.

No: Psychiatrists are ethically bound not to date former patients. I assume that the same goes for psychiatric nurses. Perhaps inquire with the american nurses association.