Most people I've known have had issues, with alcohol, depression, drugs. or bipolar, or schizophrenic. my father was a therapist so its never been surprised or frightened me. well, almost never. a couple times it was terrifying.
as you say, there's a ton of cookbook advice online. you are excellent to try and help your friend, but yeah, you can't do it for them. they have to want to get out of the place they're in.
with any mental health issue, the danger is that when you get to a dark place, you completely forget that there is light. after years of dealing with it you may remember 'oh, this is that thing again, I have to wait it out.' but that doesn't always happen.
when I get depressed, my world gets very small. the plans I have in life disappear, the things that make me happy I've completely forgotten, and the jokes that make me laugh I can't remember any more. that's when I know I have to stop what I'm doing, if I can, and physically leave the situation I'm in as much as I can. physically do something different.
Pain will make you roll up into a ball and not see outside yourself. anxiety will make you stupid and confused. you almost have to trick yourself out of that point of view, with humor, change of scenery (literally-- leave the house), or running to something that is always good for you, like any hobby or activity that is always productive (cooking, clean your damn room you slob, doing the dishes, mowing the lawn, riding your bike).
The problem I have seen in others' who have stopped drinking or using drugs is that they may not have anything to do after that. they have emptied part of their life, and if they don't fill it up again they will continue to feel empty. I did have a friend who quit drinking- then he got an apartment above a bar, on purpose, so he could sit in the bar every night, year after year, grinding his teeth and wishing he could drink. he has technically changed his life, but nothing inside him has changed. you see?
practical advice for you w your friend:
-if they're not seeing a therapist, get them to. if they don't want to, then go see a therapist yourself, and report back (most people have a b.s. idea of what a therapist is a therapist is someone who listens to anything you want to say, and you feel much better afterwards. they are not ppl who tell you whats wrong with you, or try to change you). a lot of them are free, some of them are cheap, some will cut their price if you ask.
-don't wait for them to ask if you want to hang, and when you do hang, don't just sit there. do something different than you usually do- if you don't go to the movies, go. if you don't hang out in the park, try it. if you have an errand to run, ask them if they want to go with you.
-if you've got a sense of humor, use it. humor takes people outside of themselves. humor is THERE for people to relieve themselves of horrible things in their own lives.
-forgot- don't try to lecture or change them. as you say, they can take it the wrong way, and really it *is* the wrong way. it's their problem, the best you can do is be there.
-also Red Frank ingame - and Mad Frank (ultimate ironman)-