If it is of intellectual curiosity, the best thing would be to read about the disorder in question. The problem of maladative behavior, 6th Edition. Clinical and scientific perspectives, 2nd ed. It's not necessary to know a lot about a disorder in order to help somebody suffering from a problem. Knowledge can help to steer away from pitfalls, on the other hand, it can turn a friend into a stereotype.
Diagnoses are troublesome to begin with, and many psychologists are wary of using diagnoses at all.
It seems that the medical model of finding the cause of a problem and then curing the problem by treating the cause does not work with many psychological conditions. Introduction to psychology, 11th ed. Fort Worth, Tex. Social psychology, 2nd ed. A history of modern psychology, 5th ed.