Incidentally, I think it's not just unethical, it's potentially harmful.
It's not unusual for people with mental distress to have problems with trusting people. That can range from having PTSD following, for instance, abuse in a relationship, through to a reality not spoken of (in my opinion) enough in our society about how people perceived as vulnerable are more likely to be targetted by cheats.
I have a theory that one reason why people in certain social groups are more likely to end up with paranoia is a reality that people in those groups are more likely to have been cheated, exploited, betrayed etc.
So whilst the person in question may have thought it was an ok thing to do, it had the potential to be extremely damaging.