Careful with this, it's easy as a partner of someone suffering from this to over time get too impacted by the constant depression talk and walking on egg shells to not say the wrong thing.
My recommendation is to help your friend or partner get professional help, if after a couple of months of listening you're still just listening to same stuff. Otherwise it'll take a toll on you and two depressed people are worse than one.
Anyway, I think friends and even just acquaintances can do more, even if that seems counterintuitive.
It's a simple algorithm. Depressed? Build.
Not really
Please do not do this. Suicide and being drunk are very correlated. If you have a firearm in the house, or even if not, this is the single worst thing you can do for your own survival.
Also, put your engineer hat on. You _absolutely_ do not want to kill yourself with a firearm. Yeah, it usually works. But if it doesn't, you will learn what true misery is.
One of the worst things about this is the feeling that you can't talk to anyone about it, as the moment you do, you start having to take care of _them_ emotionally, at a time when you just can't. If you can afford it, a good therapist is an option--you pay them so you don't have to take care of them. How to find one? I don't know. Try several.
Here's some first aid. First, no one gets out of life alive. Usually, there's no need to hurry your demise, as you'll be dead soon enough. And as many have observed, subjective time accelerates. If you're 35, your life is more than half done anyway.
Second, before you take steps, try to remember a few things you always wanted to do. Do them first. I've always been partial to waterfalls. Mountains are great. Music festivals can be pretty cool. Go eat your favorite foods a few more times.
Third, if everything seems bleak, get wasted. Probably things will seem less urgent in the morning.
Longer term, try to find something to do, someone to love, and something to look forward to. That's the trifecta, but even one goes a long way. Worked for me.
Will your problems pass? Probably, but maybe not. The pain, though, will lessen.
What if you're dealing with someone else suicidal? Just listen and keep listening, without making yourself an emotional burden. If you can, do what a friend would do. There might come a moment, appropriate to the relationship, where you can let the person know that you'd be devastated if they killed themselves. (I think it's okay to exaggerate a bit on this point.)
Godspeed.
(I don't comment on HN, and likely HN prefers it that way. But this seems like an unusually important topic.)