I completely understand your boyfriend's and your situation. I was a full-time pastor in the early years of my ministry when I experienced seven years of major depression. It was like sinking into a deep hole. However, I didn't know it was depression because my anger at my dysfunctional family, the loss of our second child through leukemia, and church people's sinful actions was buried in my unconscious mind. God guided me into a medical retirement and to psychological help to get me in touch with my anger at him and people.
He taught me a biblical pattern that is not in our culture and of which I had no models except Job, David, Jeremiah, Moses, and prophets like Habakkuk. God broke through when I read a book about anger with a chapter on anger at God. I began to lament, that is, express my anger to God focusing on him in faith while pouring out my extreme grief about all emotional junk I was carrying around for 43 years. It took 7 1/2 months of lamenting 3 or 4 times a week before my depression left for almost 32 years now. After another month, God gave me peace instead of anger (Philippians 4:6,7).
Please suggest persistently that your friend get someone's psychological help to unearth all of his anger and anxiety. People will understand if he's open and honest about it. Then, suggest that he read the Book of Job, the Psalms, and Jeremiah's book and lamentations. Remember that even Paul lamented his "thorn in the flesh" three times (2 Corinthians 12:1-10) and that Jesus quoted Psalm 22:1 on the cross as a lament about being surrounded by his enemies the way David was.
You're correct to say that depression is an illness, and like any disease, it needs professional help. One note: Suggest that he find a therapist who will help him delve into his unconscious instead of therapies that merely try to change his behavior (behavioral science).
You might also give him for Christmas my first book that describes my dramatic experiences
in a Christian mental hospital
and my unusual reasons for believing in God, Doubtbusters! God Is My Shrink! that I share with two fictional skeptics. It's available on Amazon.com. I'll put you and him on my daily prayer list. May the Lord bless you.
Retired Pastor Bruce Leiter the Writer.