The PMHNP has no legal obligation to act regarding a non-patient adult based on a secondhand report — clinical duties run to the PMHNP's own patients, and adult third parties retain autonomy over their own care decisions.
This is correct. The PMHNP has no legal duty to act on secondhand reports of self-harm by an adult non-patient. The roommate is 19, is not a minor, is not a patient of the PMHNP, and self-harm alone does not trigger mandated reporting for adults. However, the PMHNP should address the situation therapeutically by helping the patient process her concern, discussing ways the patient can encourage her roommate to seek help, and providing information about available resources such as campus mental health services.