Amen. All good advice.
One of the truly insidious things about depression is that the sufferer often knows that it's just brain chemistry, that it's just "all in their head". That doesn't help.
Depression is not a sign of personal weakness, misguided thinking, or flawed character. It's a medical condition, like diabetes or glaucoma. There shouldn't be any moral judgement attached.
There's still a hell of a lot that we don't know about depression. However, there are treatments that help. If you suffer from depression, one of the best things you can do is to find a psychiatrist (an honest-to-god M.D.) that you can trust and work with.
And if you know someone that suffers from depression, bprater is absolutely correct. 100%. Offering someone that kind of support may be more important than you know.