First and foremost, we must keep in mind that any ESP abilities may be hijaked or even induced by higher density beings to suit their purposes, such as discussed in Malachi Martin's Hostage to the Devil. Even if the ability seems genuine and benevolent, until thoroughly tested and the user's character is refined as well as knowledge base expanded, this skepticism should be kept in mind.
Even if your friend's ability is genuine, I wouldn't suggest to rely on it by itself, but rather that your friend learns as much as possible about antisocial personality traits and behaviors. Knowledge and the ability should always be used in tandem to prevent inaccurate results.
As for testing the ability, I would suggest going to your local public university and try to look through the abnormal psychology or psychopathology textbooks to find any names of potential psychopaths, and if no pictures are available (which there probably won't be), search their names in the internet. Also have someone else assist you (not your friend with the ability, that would confound this further), and have your friend also find names and pictures, mix them with non-psychopaths, and NOT tell you which is which(further blinds, may improve results). But as was mentioned before, PSI works in mysterious and elusive ways, and if another entity is behind the phenomenon, a truly rigorous test may be impossible to carry out. So again, it must never be relied on entirely.
I hope that helped. :D
