Environmental factors certainly can drive depression in some individuals - I have gone through two periods of depression in the past, both triggered by external factors. The second was worse, at one point I was a suicide risk, this was cause by living with an alcoholic partner. Ending that relationship relieved the depression and quite quickly. I had been taking an SSRI, but I tailed off it shortly after ending that relationship with no ill effects - it really was only treating the symptoms, not the cause.

First thing to look for if someone is depressed is see if they have good reason to be depressed and deal with that. Loss of control of your life is a sure recipe for depression for most people. If there is no obvious cause, then look for biological factors