This is interesting question, and may be hard. First of all, try to learn more about depression. For example, “Undoing Depression: What Therapy Doesn’t Teach You and Medication Can’t Give You” by R. O’Connor has a very good descriptions of various manifestations of depression. I believe, it is important, that the therapist had depression himself/herself and knows it not only from the textbooks. (Disclaimer: I never had depression myself, so my input here is purely informational, and I am not a doctor or psychologist.)

It may be hard to convince a person about possibility of depression until it becomes serious. Find a person, who is trusted by the probable patient. Try to convince to undertake a simple test. The argument can be, that there is nothing to worry about. If the result is positive, at the early stages it can be treated quite easily.

The main problem is, that person with a serious depression needs a lot of support and understanding from surroundings and continuous hard work on herself/himself to cure it: Just taking pills will not help. Basically, in serious cases the whole family should be treated.