Hello, lovely :)
As I think you know, I have clinical depression following a couple of breakdowns. Forget about self-help bollocks like mid-life crisis; it's a potentially fatal mental health crisis. They rarely look like they do in the movies.
Firstly, you're right that it's great he's decided to acknowledge a problem and see his doctor. Unfortunately, he will probably downplay the severity of it when he's there. We do this, not out of pride, but because our world-view is already so grim, we see ourselves as worthless and the entire universe as similarly depressed. A healthy person, listening to us, wouldn't get what a hugely different perspective we're coming from (this includes doctors!) because we present our feelings as normal - they are, to us - and can even be funny about them. A lot depends on your GP personally, and also on the practice's catchment area; I got the most empathetic treatment in an overcrowded London district, where practitioners have more cause to understand stress-related illnesses.
I took advice to write down my real feelings before seeing the doctor. This has to be done privately: for obvious reasons, we try to hide our honest thoughts. It helps a lot when you get to the doctor's - if you find you can't talk, you can hand your mind-dump over and wait for a response. It's OK to cry (that's for him, though it's also okay for you!)
You can't mend it. Neither can anybody else, though people who have appropriate knowledge are invaluable guides towards recovery. It's hard to describe depression. It's like living in a colour-desaturated world, where it's always dark and there is no future. Horrid beyond horrid. Suicide looks like the only logical option: there is no point, just no reason to carry on trudging through the dark. I'm a hundred times better than I was back then but, in truth, I still don't see the point. Main difference is, I now believe I will!
Medication is not a cop-out, it's a lifesaver. Ignore anyone who insists you can do it with herbals and exercise. Exercise is helpful but, at my worst, I did well to walk thirty yards - and I often had to do it in my pyjamas! Antidepressants are really hit-and-miss. It takes six weeks to find out whether your prescription suits you and three or four months to tell whether they're working. The patient is not the best person to tell if they're working, unless they have a strong reaction one way or the other. It takes a good, listening professional.
Big joys can strain your depressive to the max. We'll put out all our best responses, then be exhausted by the effort for days or weeks. Tiny joys, like the ones that interest small children (a butterfly, a pile of autumn leaves, a jigsaw, tidying a row of something) are more useful because they give us an opportunity to be mindful - to exist, just for a moment, in something honest.
You can't mend it.
You are not responsible for it.
The best example I've ever seen - and I've seen a lot - of a depressive with a working relationship was a journalist, who wrote of his "black dogs" and how his family copes. His wife ignores him. She makes sure he's fed & watered, shoos him into the shower once a week or so, and otherwise shuts the door on him. He has a young child, who knows it's okay to go tell Daddy about stuff, but just to walk out again if Daddy isn't well enough to listen or play.
This keeps the writer and his wife sane. From what he wrote, I imagine his wife's doing a great job of keeping the child sane 
Depression is 'catching'. Don't involve yourself with it. If you can, stay kindly detached. If you can't, there's no shame in quitting.
I wish you both the best of luck and the best of help.