I’m someone who is depressed at the moment, thanks to a completely unexpected lay-off. Yesterday was, quite frankly, hell. So, let me answer your question from the other side.
If your friend’s anything like me, he may be kicking himself because he isn’t bouncing back and taking action. Be supportive but firm. If you’ve noticed a pattern, point it out. Clinical depression is a disease and there are effective treatments. It does not carry a stigma among reasonable human beings.
Here’s my take:
[ul][li]** If he asks for help or advice, do I have to be careful what I say or how I say it?**[/li]FairyChatMom, if you talk like you post, I’d say your talking like you normally do will do just fine. Be alert for the usual signals, of course. Your friend may have some hot-button issues which really do need to be addressed, but if he reacts strongly against them, don’t push it.
[li]Would it be wrong of me to offer unsolicited guidance?[/li]Given what I’ve seen of your advice on this board, no. Just don’t let it be the only thing you two talk about. It gets boring after a while! 
[li]**Should I just be a sounding board, or will that come across as being patronizing? **[/li]Your friend’s mileage may vary, but sometimes talking about what’s wrong, bringing it out into the open, lets me get deeper into what’s wrong and work at it a bit. Again, I know nothing about your friends history, but when I’m in a depression, it brings back old feelings that no one was listening to me, no one could hear me. Serving as a sounding board is one of the best things you can do. Better yet (and to help your frustration if your friend’s on about the same issue for the dozenth time), ask a few probing questions.
[li]**Should I try to cheer him up or let him cry on my shoulder or distract him? **[/li]All of the above as needed and as appropriate. Laughter, even it hurts, is healing, as are tears. Distractions, sometimes, can serve as a marvelous way of getting the person out of a cycle. Last year a friend of mine was stuck in a depression, and talking, comfort, etc. weren’t working, so I kidnapped him. I turned up at his place after work, complete with fencing swords, and informed him we were going on one of his favorite walks, after which he was going to eat a good dinner. It worked. At least for that evening.
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FairyChatMom, based on what I’ve read of you, you’re a good, compassionate person, and a good friend to have. You won’t be able to fix your friend and make him better, but you can make it easier. If you know of a good therapist, recommend one. If not, stand by him while he finds one (I called 5 therapists before getting the one I have now, and you would not believe how frustrating it got at times!). Also, if you need help or support, my e-mail address is in my profile. Please feel free to use it.
Good luck, and take care,
CJ