Actually, OP, I think you are being too quickly dismissive of @Anordinarymum's suggestions. I speak as someone who has suffered severe depression. It turns you into a very selfish person, especially at that age, and sometimes you need your 'helper' to cut through the fog and say: This is hurting me too! I'm wrung out. I can't do this any more.
I do know there's a very fine line, and it's not fair to say to a depressed person 'pull your socks up' or 'snap out of it' because we would if we could, if that's all it took. But equally, in my long experience, I have come to recognise it as a battle. You have to fight the beast, really hard, or it does take over. And the only way to do that is in tiny incremental steps.
It will help her to be given a name for the way she is feeling. To be told: this isn't you. It's depression. It's an illness and we have to fight it with every weapon available. Ask her how she'd prefer to be feeling, thinking, behaving, what she'd prefer to be doing with her life, if she could wave a wand. Tell her it's possible that all of what she wants can happen. This will pass, but together with you she has to make it pass. she has to create her self a Get Well programme.
And that programme does include an hour of sunlight every day. An hour's exercise in or outside. Small, manageable chunks of socialising each week. Some mental health training, either with a counsellor or Self Help if she prefers to start privately. And two other things, both of which can be crucial to healing.
One is giving to the community in some way - helping out in some charity, planting community gardens or litter picking or helping at an animal shelter - some way in which her contribution helps someone not in any way linked to her.
And the other is doing things that might be fun with zero expectation of them instantly curing her. Just doing them for the sake of it, because she might discover something that sparks her interest. Don't push too hard on that one. She can try very easy things first like watching a box set or listening to music from a different genre or era. But bit by bit she might try a new sport or hobby or get a PT job or care for animals while friends are on holiday etc. Just to show her she is valuable in the world. She matters. And the world is full of things to explore and do that will give her meaning and joy in time.
It's important she feels under zero pressure to feel happy at any point during this healing period. She's allowed to feel sad and angry - valid emotions. But ask her to honour her obligations to herself to give it a try - starting with very gentle, easy things. You could help her make lists of easy things, mid range things and things she'd like to do but doesn't feel up to yet so she sees her progress.