Saying someone appears mentally unstable is not the same as saying someone is mentally ill. For example, you may say a person who is confused and irritable is mentally unstable. That does not mean they are mentally ill.
Low blood sugar, anemia, carbon monoxide poisoning,alcohol withdrawal,swelling of the brain, a concussion,subarachnoid bleeding,a skull fracture, low salt in body fluids,or drug allergies can all cause confusion and agitation. They are all considered physical illnesses.
Unless the POI told A. C. I that they have a mental
illness or somehow A.C.I obtained medical records on the person we don't know that the person has a mental illness. We just know that A.C. I's perception of the person was that they were mentally unstable. That could mean many things to different people. What does that mean? To one person someone who dresses bizarrely in mismatched clothes all the time may be deemed mentally unstable. In reality maybe the person is color blind and can't tell that their clothes don't match.