I am on the board of officers for my college's GSA, and we are all very close. Most of us are in some way not cis, and I think all of us have some mental illness or another. Each of my friends there sort of needs something different when they have bad days with their depression.
Most recently, one of my friends I offered to bring some board or card games over or do a puzzle. They're not a big physical person and if you spend money on them they're uncomfortable. The best for them is doing a fun, free activity to distract them and get their mood up for a bit.
Other people, we have long talk sessions. Just about anything but it'll go naturally to what's upsetting them and then back to other things and back again, so it's not forceful, it's just a natural flow.
It always cheers me up a little bit when a friend knows I'm upset and thinks about me enough to get me something I like. Once my roommate brought me home a king cone and it's such a small thing but it really helped me to know there are people who care even when it seems like everyone doesn't. (But as I said above, this makes some people feel worse and guilty, so use your judgement based on the person.)
If you can find a way to get both you and your friend to take the love languages test, I find that very useful in learning how to brighten someone's mood. If they're a gifts person, that small candy bar gesture may help a lot. If they're a physical person, being there for a big hug. Adjusting to how they understand love is more effective than just going off of your own experience (though own experience still definitely helps!)
And I know "brightening someone's mood" is only a tiny dot in the grand scheme of depression. It's a lot more than just that. And as much as we wish we could cure it, it's the job of a professional. The best we are able to do as friends is to be there and help them feel as good as they can. You can suggest to them that they seek help when they say something to you that is beyond your knowledge, but don't push it too hard if they don't want to. Small things to cheer them up and helping them distract themselves when they're in a bad place can still go a long ways in reminding them that they're loved and cared about.
I remember I was doing really badly at the beginning of the summer because for me, it was like I kept having good plans to move forward with transition, but they'd always get ripped right from my fingers. Top Surgery was no different. The worst thing anybody ever said to me was "it will happen eventually." Eventually doesn't mean I'll for sure be around to see it. Every time it gets ripped away from me it's harder to see it becoming a reality and I stop getting excited for any small movement forward in preparation of it being taken away again. Some people do appreciate comments like "your day will come" but exercise caution because it's easy for a brain in a bad headspace to twist that and make it feel worse. This is really true of anything to be honest, but it's such a common phrase that I wanted to mention it.
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